The rainy season usually isn't bad at all in the June/July timeframe (mostly single late-afternoon downpours), BUT it can be more pronounced on the Andaman coast and in the north and northeast, where humidity can also be high.

The interests given sound very Chiang Mai-ish, and it's pretty cheap to stay there and there are trekking opportunities if you're into that. I did a pretty good one back in 1996 from Lek guesthouse, I think it was, to the Karen villages near the Burmese border. CM is a favorite for language courses, cooking, etc. (dunno about yoga but would think it's bound to be there. Hotels can be excellent value and I usually go for faded 3-starts and try for the $30-ish range, with aircon, of course.

Beach-chasing gets progressively harder because they can't stop overdeveloping (even national parks aren't safe from that). Airlines like Nok Air and Air Asia have plane/ferry packages to islands in Trang province, though that's the Andaman. The old standards of Phuket, Krabi and Samui have gotten awfully crowded. Ko Pha-Ngan could be an alternate base on the gulf side. Ko Chang is built up but not quite as bad as the big 3, and there are islands in that vicinity with less development. Ko Samet used to be a good choice but IMO is overdeveloped now.