Though gambling is mostly illegal in Thailand, the horse racing track behind Rama IX lake allows betting on their weekly horse races. You're unlikely to win anything without a "connection", but it's cheap and makes for a good lazy Saturday.
It's really the spectacle that makes the races worth visiting. Every Saturday scores of working-class Thais bring their families, a cooler and whiskey to the track, set up camp on the bleachers and pore over pamphlets of esoteric horse knowledge. Everybody rushes to the fence when the gun goes off and back to the monitors as soon as the horses cross the finish line. Betting is inscrutable, with the odds changing every minute up until the race starts. We put ten baht on number 4 and waited for payday, but several races later we were up one baht and tapped out.
As we said before, gambling is illegal in Thailand, but the horse races operate above board because they're owned and run by the military. People On 'the Internet' claim the races are all fixed, which is not difficult to believe, and that the best way to get tips is to ply highers-ups with whiskey until they're drunk enough to just straight up tell you who's going to win.
If you need something to do on Saturday other than curse the night before we highly recommend it. The track is here and races go about every 20 minutes from 12:30 till 5:30. Betting is all in Thai but it's not difficult to figure out.