Having lived in Thailand for 3 years (sadly back in NZ now), I think it would be quite possible to relax in BKK - subject to whatever your key criteria for 'relaxation success' are!
Marriott Resort or, perhaps better the Sukhothai, are worth considering...the hospitality is Thailand is generally delightful!
http://www.sukhothaihotel.com/home.php
http://www.marriott.com/hotels/trave...esort-and-spa/
Cheers, TK