Cheesecake Factory in the Galleria? There is a downtown area of sorts in Houston, connected by tunnels, and a tram system above ground. The Aquarium is another option I guess. There's also a public service express coach service from downtown to the airport.

I would not turn up too early at IAH, it's surprisingly sparse as terminals go, and the lounges aren't so exciting. They do have mega TVs for compulsory viewing, however.