what I can recommend is Ko Phangan if you want developed island but still not completely ruined by tourism, but if you really wanna enjoy small islands then check Ko Jum and Ko Chang on western coast, have fond memories of both of them, one of them has no cars, no power lines, actually maybe both, their quality of water ain't that amazing as some other islands but it's well compensated by relax
but personally my best experience from half year spent in Thailand on multiple visits was Mae Hong Son loop (preferable start in Pai), stunning landscapes to enjoy from seat of motorbike and also great relax around Pai, reserve at least week for that 700km loop, though it can be done in 4 stages but you don't want to rush especially in Pai
the oldest national park east from Bangkok is also quite nice, though it's better to go there during workdays when it's not flooded by all Bangkok, but you can find real jungle with leeches on your legs and get lost
my another favorite place is Kanchanaburi with great night market and nice daily motorbike trips to Hellfire pass with Death railway and other day Erawan waterfalls
actually the sea in Thailand ain't that special, if your eally are into amazing beaches then better go to Philippines or Indonesia
btw. summer is rainy season, better go there from november-april, in summer would be better to go to Perhentian Kecil on northeast coast of Malaysia, that's amazing place with pristine clear water, cheap diving, amazing life under water even with snorkelling including small sharks (not the dangerous ones) coming right to beach close to edge of water, plus on western coast you can head to food capital of world in Georgetown on Penang island plus enjoy Cameron Highlands with great trails for hiking in jungle or enjoying beautiful tea plantations plus touristy but pretty Melaka and diverse KL, both countries are great