I got back from my trip on Friday. I agree with Colin, pack little. I had hand luggage only, clothes wise I took - 7 tshirts, 7 pairs underwear, 3 shorts, flip flops, espadrilles (never wore), 1 pair of trousers, 1 lightweight coat (never wore) and it was plenty (you pay about £2 for laundry). Other stuff I brought were headphones, chargers, guidebook (used my phone most of the time), toiletries.
My biggest recommendation is to buy a sim with internet access, it saved so much faffing between wifi spots.
I did the following:
3 nights Bangkok
2 nights Chiang Mai
6 nights Pai
3 night Chiang Mai
8 night Koh Tao
1 night Bangkok
I took a cooking class in Chiang Mai from tripadvisor - it was fun although I don't remember much. I had friends who went trekking and they said it was cracking, but it does kill quite a few of your days so consider if its worth it. I also went to an elephant sanctuary (nice but pricey) and also spent all my time in Koh Tao diving which I loved.
I'm not too sure about bike routes, but you can hire a bike anywhere and most decent hotels/hostels have them for free, although consider how hot it is. I was pooped after 30 minutes. I did hire a scooter in Pai as most attractions were some distance out - I wouldn't recommend it unless you can ride one.