Single parent traveling to Thailand with a 4-yo son
 

I am a single parent, and will be traveling with my 4-yo son to Thailand in February. Planning on staying in Bangkok for 2 days, and them moving to Koh Samui. Any suggestions on where to take him in Bangkok? Thank you!

the aquarium at paragon mall, the zoo, a boat ride on the river..

I agree with the aquarium ( Siam Ocean World) at Siam Paragon center.

A boat ride on the river would be really fun. You can ride for free on any of the hotel boats from the piers by Taksin BTS station. Depending on your son, a round trip on one of those might be just the right amount of time and comfort. If it isn't long enough, you can take another one. The two longest will be Anatara Hotel (ti goes south of the pier, and River Mall to the north. If he really loves it, you can go on a ferry (under a dollar from the pier on the right at the same spot), or you could hire a private boat for a long tour.

Here are some activities that sound interesting, even though I've only been to Ocean World:

http://www.bangkok.com/fun-stuff-for-kids.htm

There are daily trips in Bangkok. If you really want to see the city with your kids - you can order from one of the travel agencies located everywhere. I also recommend taking the kids to a floating market. Great experience for the kids

With a mere two days in Bangkok, you don't really have any time for day trips. Also, the usual day trips offered by travel agencies would not be a good fit for a 4 year old.