Lists of places tell you where to stand. This one is about what to actually do once you are there, which is a different question and usually a more useful one when you are travelling by yourself.
Everything below can be done solo without arranging anything in advance, and several of them are the fastest way to meet other people if you want to.
Ten things to do
Learn to cook one dish properly
Half-day cookery courses run in every sizeable town, usually starting with a walk around the local market to work out what the ingredients actually look like. You will come home able to make one curry from scratch, which is a better souvenir than most.
Take an overnight train
The sleeper services north to Chiang Mai and south towards the peninsula are an institution. Book a lower berth for the window, bring a jumper for the air conditioning, and accept that you will be woken early by the sunrise and the tea seller.
Spend a morning at a working temple
Not the famous ones. Any neighbourhood temple in the early morning, when the monks are back from the alms round and the courtyard is being swept, gives a completely different impression of Thai Buddhism from the ticketed compounds.
Ride a long-tail boat through the canals
Bangkok was built on water and much of the western bank still runs on it. An hour on the klongs, past stilt houses and back gardens, resets your mental map of the city more effectively than any viewpoint.
Walk a night market from end to end
Resist the urge to buy in the first hundred metres. Walking the whole length first, then doubling back, is how you find the three stalls worth stopping at, and it is a comfortable way to spend an evening alone in a crowd.
Watch elephants without riding them
Observation-only sanctuaries, where animals are not ridden and visitors keep their distance, have become the standard responsible option. Read carefully what a place actually does before booking; the wording on this varies a great deal.
Snorkel off the Andaman coast
Day boats run from Krabi, Phuket and Koh Lanta out to the smaller islands. Even a beginner trip with a rented mask puts you over reef within an hour of leaving the pier.
Time a visit around a festival
Songkran in April turns the whole country into a water fight; Loi Krathong in November puts candles and floats on every river and pond. Both are chaotic and both are far more fun than they sound when you arrive without a plan.
Trek a day in the northern hills
Out of Chiang Mai or Chiang Rai, a single-day walk through forest and terraced fields is enough to see the northern landscape properly without committing to an overnight trip.
Eat standing up
Street food in Thailand is a functional part of everyday eating rather than a novelty. Pick a cart with a queue of local customers, point at what someone else is having, and eat it where you bought it.
The common thread is that none of these require a companion. Thailand is one of the easier countries to enjoy alone precisely because so much of daily life happens in public.