Without a doubt, Bob's Steak and Chop House is hands down, bar none the best steak....if not overall the best food....available in Dallas. And no, it's not at all uncommon to see the local professional athletes eating there as well. If you're coming to town, I very highly recommend it.
As for things to do? Ehhhh, I recommend sitting around in the AC drinking beer and watching sports....it's too hot until mid september for much of anything else.
But if you must, Dealey is neat for a first timer. And right next to it is the Old Red Courthouse museum that just opened, and it's pretty neat if you're into history. Rangers Ballpark in Arlington is a very beautiful stadium if you like baseball, and right next door you can see the construction on Jerry World. A tour of Texas Stadium is pretty anti-climactic, but alright if you're a diehard.
Nightlife is going to be best around Victory (can also see the AAC there) or on Greenville Ave. if you want to try and bring a 19 year old coed home with you.
I would suggest a day in downtown Ft. Worthless, theres a lot to do there to kill a day, unlike downtown D.
edit: Oh, and you happen to be a Pantera or metal fan, their strip club The Clubhouse is a must, as is Dimebag's grave (in Arlington). Most weekend nights you can grab a table right next to Vinnie Paul and whichever metal stars are in the area that weekend. It's a relatively cheap strip club with some nice looking ladies.