if i can give you 1 piece of advice it'd be to visit the full moon festival in koh pangan.
it's a way less touristy island full of natives and expats. it's incredibly cheap and chill. though from what i've heard from friends this festival is just pure mushroom and acid craziness. last time i was in thailand i was travelling w/ family so i didn't go there, but a bunch of my friends have and they said it was balls to the wall insane.
also be sure to visit pattaya and plan to spend at least a week in bangkok. there's just so much to see.. make sure you don't just stay in khaosan road the entire trip, try to branch off a bit. if you're feeling real fucking bold go to a restaurant that is clearly for thais only. if they don't have an english menu you're golden. order tom yum goong and start sweating. they have all kinds of shit there.. tripe, pig's placenta, chicken hearts, scorpions, various lizards, insects et cetera.
other dishes you absolutely have to try: papaya salad, beef salad, giant tiger shrimp, thai ice cream (tamarind is my fav, try them all though), cabbages & condoms restaurant in bangkok, crab in krabi..
njoy your trip m8, thailand is always a great choice.