There are lots of choices in Thailand. Talk to fellow travelers; they can make recommendations. Or, if you like the people running the hostel or hotel, ask them to make suggestions. Shop around. Get to know prices. Some offerings are better than others. You can also try to haggle and get the price down, or have something else thrown in. There's no harm in asking. In fact, I recommend that travelers always ask if they need or want something. Ask in a nice way; and there's a good chance it will be fulfilled.
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" How do you guys go forward to the random activites??"
Most hotel front desk people are friendly and can provide useful information. Some hotels have brochures and other tour information. Some hotels have tour desks. In Thailand, unless you are in a very small village, most cities popular with tourists will have travel agencies that provide tours, expeditions, adventure type stuff, etc.
Normally you choose what you want to do from a tour book or other information at the office, get prices and pick up times, pay up and next morning the tour company picks you up at the hotel front door. They bring you back at the end of the day.
(Always keep your receipts. I have never been cheated by any travel agency, once or twice they have been a little late picking up. Being late is usually because other tourists they must pick up were not prompt! In Bangkok when I hear about problems with travel agencies usually they are in the Khao San Road area.)
Travel agencies in Phuket, Bangkok, Pattaya and Chiang Mai routinely provide tours, etc. Just find a few not far from your hotel and see what they have. These are "shared tours" where you will be picked up with a van that seats 6 to 8 other tourists. You do meet other people on these tours.
Look into: "Flight of the Gibbon."
Use Google for searching what is available for the city you intend to visit. Your hotel front desk can point you in the right direction.
[ Edit: Edited on 13-Feb-2016, at 14:46 by karazyal ]