Avoid Pattaya. In nearly a dozen trips to Thailand, I have. And I don't feel like I've missed a thing. From what I understand, it has a vaguely pleasant beach but that's about it, apart from the sex industry (the city became what it is today as a result of being the leading R&R destination for American soldiers in Vietnam). Thailand has far more pleasant beaches, some ranking as world-class destinations, for which Pattaya is a very poor substitute. Get your Thai beach time on another visit, when you can do Koh Phi Phi, Krabi, Khao Lak, Phuket, or Krabi. Also, you need to allow much more time for transport than the Bangkok-Pattaya distance suggests; even with the elevated highway, you won't be doing 60 mph for the whole ride.
A day or day and a half is very little time in Bangkok, especially on a first visit.
This thread, in the Thailand forum of FT, has a robust discussion of must-sees in Bangkok on relatively short visits or 3 days or so. It should be a handy guide for you.