The closest thing you will get to the real thing around Bangkok, this theme park revolves around a man-made beach and lake. Along the American model for these sorts of thing, the lake has artificially generated waves and sand. Other attractions include a fair with merry-go-round/carousal. This is another all day event, so unless you have plenty of time of your hand, it's not worth the trip. If you do, it's almost obligatory!
Details: Siam Park is located north of Bangkok. Admission for adults is 400 Baht and 300 Baht for children. Opening hours are 10:00 to 18:00 daily (09:00 19:00 at weekends).
How to get there: Again, because of the distance involved, and the fact that you are probably taking a child, a taxi is recommended. Although expensive, the savings in convenience entirely outweigh the cost.
Even better, a number of hotels and travel agents organise tours to Siam Park. If you count up all of the costs they often work out cheaper than going yourself and are well worth considering.
The following buses will get you there:
Bus number: 26, 27 (from Victory Monument)
101 Sukhapibarn 2 Road