Spent the summer before my senior year (2012) at state in Thailand. A lot has probably changed.
Ayutthaya (hour train ride north of Bangkok) has beautiful temples, and not crazy crowded and filthy like Bangkok. Spent my first week there teaching English.
Chiangmai is pretty far north, but the elephant and tiger sanctuaries were worth the trip. Ps I don’t care if they drugged the tigers, it was really awesome.
One night in Bangkok is all you need to catch a boxing fight and a wild as hell sex show(picture turtles and birds going places they do not belong). Maybe send the wife off for a spa treat or something for that outing. You could make a long day trip or overnight from Bangkok to Phnom Penh, Cambodia. Really shady border crossing, but Angkor Wat is worth the $20 (2012 dollars) bribe for the smooth crossing.
Then head south to the islands. Pick your poison there. I chose Phuket, then after a few days of hard partying took a day-ferry to koh phi phi. That turned into a two-week booze and other thing filled haze on an island the size of the cotton district (damn I miss that feeling). Accidentally overstayed my visa (like a $50 fine) and made my way to Singapore, via Kuala Lumpur. Didn’t have any money left to enjoy those two cities, but they sure were nice.