The nearest beach that has ease of access is really the Pattaya stretch although there are some closer beaches but they are close to quite industrial ports so not really that attractive.

There are probably some southern beaches that would be around 2 hours but the transport options (unless you have a car and driver) are not as frequent as the Pattaya options out of Ekami Bus Station.

You can get to Bangkok from the Pattaya area by car in around 2 to 3 hours but almost half of that will be through Bangkok traffic.

Bangkok is a nice enough city (at least I like it) and the beaches are close enough to get to on Friday evening and back early Monday.

Personally I would stay downtown unless you can find a hotel on the BTS or underground system. Travelling times in Bangkok are horrifiic by car.

Hope this helps

Stekmer