Also consider, in/near Tarrant County:
NRH20, which is just as good as Six Flags' waterpark for half the price and has a contest associated with it,
Cowboys Stadium, which has all the art exhibits and offers tours - it's actually very interesting,
Great Wolf Lodge, pricey but has an indoor water park, so you're guaranteed to be OK on a rainy trip day,
You can also drive up to Denton and do a concert at the Winspear, since UNT has a semi-invitational every year.
There's also a "contest" that performs every year at the Majestic in Dallas, and is a great old-time theatre. I think American Classic hosts that one.