Winstar is 18 and up, and has a 46 table poker room. There are buses that will get you there for cheap + give you a card to play slots for most of the amount you pay for the bus. I have seen people mix and match buses to stay longer, but you should ask the bus service about that.
Here is a bus shedule:
http://www.casinoexpressbus.com/Winstar.pdf
Other than that, Mavs, Stars or House of Blues. Hang out in the West End, Greenville Avenue, or gritty Deep Ellum. West End is more 18 friendly.