In terms of nature stuff and sightseeing, Houston isn't really the place for you. Houston is really all about eating. The food here is spectacular. Tell me what you're into and I'll give you a list of restaurants as long as your arm.

Still, there some really interesting things to go see, but they're out of the way. One cool destination is Brazos Bend State Park, where you can see alligators hanging out all over the place. Lots of awesome wading birds. Very nice. If you like to see massive industry, you might be able to arrange a boat tour of the shipping channel. Photography is forbidden, but you can see lots of refineries and industrial stuff. To borrow a phrase from Jamie Zawinski, it's the grim meathook future.

I agree that Spacecenter Houston is pretty much a kid playground, not an adult destination. What you really want is to have a friend who works at NASA JSC to give you an inside tour.