Lonely Planet: Thailand aka The Backpackers Bible, refers to Tak as "particularly uninteresting", only including it in the book because "you might get stuck here".
I first came to the city intentionally (if only to meet up with some friends), but ironically, did actually "get stuck" there my second night, after finding out the buses didn't go to Mae Sot on the King's Birthday. Fortunately, Lonely Planet's review of this city may be an unfair write-off of hidden gem in Northern Thailand. While it doesn't have any outstanding "sites" per se, it does have some scenic river views ...and frozen cocktails. Throw in some bicycles to scoot around in, and the company of 4 wonderful people, and this city is worth more than a night passing through....
Whatever reason brings you to Tak, be sure to stay at the incredibly character-building Mae Ping Hotel. You might recognize the bathrooms from horror films such as SAW I-V and Hostel, as I'm pretty sure they were all filmed within these drippy, rustic tile walls. * Also, it's good to know that if the villians from these films ever did show up here, the rock-hard pillows could be used as lethal weapons.... seriously.
Those late nights watching Iron Chef at 4am in America will pay off when you visit Hot Pot the next afternooon for an endless buffet of creative soup conoctions - boiled cow stomach, bok choy... maybe some sushi... then of course don't forget the redbean and corn desserts. If you can even bare to stand up after this feast , exit the back into ... the Arcade!!! Watch out for air-hockey pucks that come flying towards your face.
Later, use the rickety Indiana Jones bridge to cross the Me Nam Ping, drink some beers on the "beach", shoot some fireworks if you got 'em, and watch wild horses toss their trainers into the sand.
Steal some bikes from your friends apartment to visit lesser known sites such as the Shrine to King Thaksin, the nameless road-side vegan restaurant (!!!), and then sit by the river and watch the Thais play on the newly-famous Tak tree swing, or effortlessly catch gigantic fish for dinner without so much as a fishing pole.
After the sun goes down, wander around the Night Market for an authentic eating and shopping experience. And while every city in Thailand has a night market to peruse- not every city has the friendliest roti lady in Thailand, or a VW Bus serving mysterious coffee drinks out of its side door.....
When it comes down to it, I guess it's the simple pleasures, the hot food, and the good company that allows you to open up and enjoy an otherwise average , middle-sized city in the center of Thailand. But next time you're traveling to Mae Sot, Um Phang, or Sukhothai... forget what the Bible says... and make sure to "get stuck" in Tak.