I love Thailand and highly recommend it. Just make sure you do your research on where to go.
Beach wise I'd keep away from the tourist hot spots (Koh Samui, Pha Ngan etc) and head for a small island like Koh Samet, it's beautiful. Koh Chang is also very nice. If staying on the mainland the towns + villages around Krabi are worth a look. Keep as far away from Pattaya as possible.
I'd base yourself in Bangkok. Plenty to do that isn't seedy (your Mrs will love the Siam Centre + all the Centrals!). All Islands and places of interest are easily accessable from 'Bangers' by road or air for not too much cash. Once again I'd keep away from anything too touristy. Places like Khao San Road are worth a look for an hour or two but pretty dull compared to what else is about imo. Public transport's great for getting around. The Skytrain is safe + clean and can take you all over the place. You'll never be stuck for a taxi and you'd soon realise which are the legit ones.
The floating markets at Thonburi are well worth a visit and can be done in a day trip from BK. A couple of days around Kanchanaburi is worth it as well. Go see the bridge over the river Kwai and visit 'Hell Fire Pass'.
Loads more to do in Thailand, it's a wonderful country. As long as you make an effort with the locals (learn 'hello', 'thank you' etc) you'll find out why it's called the land of smiles.