I'd advise a trip up to Chiang Mai as well. Great night market, fab food and you can go out into the mountains from there- elephant trakking, up to the hill tribe villages etc.- and a welcome contrast to the madness that is Bangkok (which is my favourite city in the world!)

As for beaches it depends what you like- personally my favourite was Cha Am/Hua Hin but Krabi area is also great.

I was going to suggest Chiang Mai as well but note that it is still a large city so can be frenetic much like Bangkok. Beautiful surrounding countryside though and it is possible to stay in resorts in the country rather than town. Another area might be the Golden Triangle near the borders of Burma and Laos.

Beach wise I would consider Phuket or Krabi. Phuket has a number of resorts on it so you should find one that suits your needs from lively Patong to very quiet beaches like Naithon. Both Phuket and Krabi offer easy access to the famed Phang-Nga bay area where there are numerous islets you can take boat trips too, for beaching and various water based activities.