As usual everything starts at the airport upon your arrival! If you arrive to the new Bangkok Airport ( Suvamabhumi - BKK), you face with a stunningly modern and majestic terminal. Normally, everything goes smoothly at BKK up until you try to get a taxi. There are some stewards directing you to the taxis going to downtown, like in JFK. They normally ask you the destination. In this position, I highly recommend you to ask the ballpark taxi fare INCLUDING the highway tolls. Because you don't notice how much they charge, in the dark toll collection plazas in a language you don't know at all, for the highway (around 70 THB normally). The drivers might charge as much as the meter count (between 200-400 THB) for the highway which is not the actual cost obviously. So just for precoution, double check the taxi fare and make the driver accept that price before get on the cab. The other fact about the taxis at BKK is that they are extremely old cars. So don't be surprised if you get in a car which makes you fell it will stop in 20 meters.
Among all others, the next taxi trick you need to be careful about is the meter trick in downtown cabs. When you get on a cab, be sure the driver starts the meter before moving the car. This mostly happens in the night. In order to avoid that you should specifically ask him to start it. Otherwise you may find yourself in an unnecessary and tiring discussion and finally you most probably pay much more. I can give you my experience as an example. I was going to a very close location during the night and get one of the cabs in front of the hotel. He directly asked me 200 THB and I asked to start the meter otherwise I will get off. After a very long discussion back and forward, he finally did and the total fare was 70 THB. So you do the math. As usual, you need to be careful with the cabs during the day as well! For instance, you may want to go a historical place like Royal Palace. They have a tendency to drop you at the entrance/exit where the conners are populated. So double check where they leave you and don't believe anyone comes and talks to you fast. You actually feel it because an average Thai, even in tourism sector, can't really speak that decent and fast english unless he tries to con you! The best way to avoid is to check the opening and closing hours of the monuments for that specific day and only talk to the people behind a ticket desk or something similar. The last but not the least tip about the taxi/tuk-tuk transportation is the "gem fraud". Most of the people know that the gem prices are extremely low as most of the them are mined and traded in Thailand. However, the fakes are also in sold. I can say there are much more fake gems than the real ones on the market. In Bangkok, the drivers are like the middle men. They usually mislead the tourists who ask for a reliable seller. Anyway, I don't understand why people ask for something to the drivers who is the most anonymous person. So they usually take you to a temple where you meet a "monk like" guy who sells the fake gems. You might be put such a good and trustworthy setup and eventually you may spend quite a lot of money, like order of hundreds of euros. So you'd better not to talk or ask anything to a driver and don't disclose any personal information if possible, like where you stay, who you are etc.
After this short pause, coming back to the airport transfer issue, of course there are other options to get out of Bangkok airport. A frequent train line commutes between the airport and Bangkok. You can select either the express line or the city line depending on the location of the final destination and speed. The express line goes Makkasan station non-stop. Whereas, the city line have several stations on the way toward its final destination Phaya Thai but it also passes by Makasan station. The final stop prices of the express and the city lines are 150 THB and 45 THB (varying between 10-45 THB), respectively. Makkasan station is very well connected to various locations downtown by metro lines. So my recommendation would be taking the fast line and metro combination to reach the final destination safe and quick as the price difference is minor. For the metro, the wisest thing would be enjoying 24h daily passes if you will travel in the city. The multi level - air conditioned "sky train" metro line is very well operated and saves substantial time as compared to walking.
Taxis and tuktuks can be another option to get around the city, especially the old town located by the river. But one needs to be smart while getting a cab as I have explained above. Taxis may cost between 50-500 THB which is around 1.5 to 13 EUR or 2 to 17 USD. So if you want to get somewhere quick they are good options unless you stuck in to a traffic jam like above. Besides there is a bus service available in Bangkok but I don't recommend it due to obvious reasons like pickpocketing, traffic jam, jam packed busses and hot/humid weather!
You solved your transportation! Now, it is time to wonder around... I won't mention about the historical places and must see location that you can find in any tourist guide or on any blog about Bangkok. What I will tell you is that this city is a shopping paradise! You can find anything and everything in the city for a wide price range. But there is one rule of thumb for Bangkok: Always bargain and don't buy anything for the tag price!
The "central" part of the city (around Nana, Phloen Chit, Chit Lom and Siam MRT stations) contains numerous shopping malls. Among all, Gaysom and Central World Plaza (between Chit Lom and Siam MRT stations) offers the most luxurious and relatively high-middle class shopping experience, respectively. Ploenchit and Big C shopping centers are more focused of middle range goods. In this places don't hesitate to bargain at all, especially for the souvenirs. Finally, Platinum and Palladium shopping centers on the river bank offers very cheap and wholesale type stuff. I highly recommend you not to miss all the vendors and hawkers just outside of those shopping malls. You might find the very same goods on the stands for half price! You can also find open markets, where you can find mostly clothing and food, at some fixed locations in the city. I recommend you to check them out to mixed with the real population of Bangkok.
During all that rush, don't forget to hydrate and eating. There are basically three main options to eat in Bangkok: eat on the streets, check out the food courts of the shopping malls and go to a fine restaurant. Frankly speaking, I didn't do the last one and don't recommend it because it is pricy for such a cheap city experience. Moreover, they offer foreign food which is not really special. The first option is actually very tempting but you need to have an insensitive stomach. if you have one that is the most tasty and cheapest option. You can have each course for less than 2-3 EUR. The intermediate option serves the best for the ones who don't want to spend so much on food and also don't want to risk the holiday much. Each and every shopping mall have a huge food court where you can find all types of asian food and also some global chains. The variety of the food is actually makes you have very hard time to select your meal. In this places, you can fix your lunch or dinner for maximum 10 EUR including drinks, dessert and a very large meal.
Overall, Bangkok is a great metropolitan city which offers you all types of entertainment! So you should just do two thinks on your side to enjoy the city: get your plane ticket and spend money. Have a great travel and enjoy your stay in Bangkok. Don't for get to STAY SAFE!!!!!