The highway into Bangkok from the outskirts are subject to tolls. If you get into a taxi and the driver asks if you want to go on the highway, you will have to pay the toll. It is normally quicker to go on the highway, and toll isn't much. I normally gave the driver the money when we got to the toll booth, although if you don't, he will add it to the metered bill at the end of the trip.
The only trap that you might fall into, is being taken on the highway when you don't need - ie. being taken on a longer trip than you need to.
Unique Suggestions: Try having a look on the map, before making the taxi trip, and see if the route will go via the highway.
Fun Alternatives: If you can - try using the buses. They aren't always very inviting - but they are great fun. You pay a conductor once on the bus. There is a good website - www.bmta.co.th, which gives the route for each bus number. Most of the routes seemed to terminate at Victory Monument, from there you can get the Skytrain to most of the popular parts of the city.
Not really a Tourist Trap, but you better be warned. When U tell a taxi to go by the highways (to avoid traffic jams and get there sooner) you must pay the highway toll (about 40 Bahts, 1 USD) when it comes, not the taxi driver.
So you must add that amount to the total coast of the ride, remember that!