I've lived in Houston for decades and can proudly say that it is a great city in which to live and work, but not much fun to visit. The best intentions of people who really love the city haven't been able to change that much despite a lot of effort. This is because the geographic and climatic location of the city just isn't that great for sightseeing -- we don't have dramatic mountains or a pretty coast line, and the summer is brutally hot and humid.
With that said, we have great weather in the Fall and Spring, and the people are extremely friendly and welcoming. We have a very diverse population in terms of ethnicity and culture and, relative to many other places in the South -- or anywhere else in the world, for that matter -- have made something positive out of that diversity.
If you're interested in natural phenomena, there's subtle beauty here. Houston sits at the intersection of three major biomes -- the "Piney Woods" of east Texas (to the north and east of the city), the broadleaf central Texas biome (to the west and northwest) and the coastal prairie (to the west through the south). The latter, especially is of interest to people who love birds, since the major North American flyways for migratory birds either have their terminus just west and south of the city or pass over that area (the so-called Katy Prairie area). In November, you'll see tens of thousands of gorgeous migratory birds flocking into and through the area, including all the major species of geese. Out in the Katy Prairie you'll see enormous flocks of Canada Geese, Snow Geese and other kinds of birds. This coastal wetlands environment is in a state of robust health and, if you're willing to get up before dawn to really enjoy it, you'll see flocking behavior that will take your breath away. I've got to say that some of the most profoundly peaceful and beautiful things I've seen in my life have been in a cold goose decoy lay out there just west and southwest of the city.
If you want to fish, we have some of the best big game fishing in North America along the coast and just offshore. The offshore drilling and production activity has resulted in an artificial reef habitat that has created a fishing wonderland.
In the city itself, we have a pretty good nightlife scene -- or at least the youngsters I work with tell me so. If you want truly awesome Mexican and TexMex food, go just east of downtown into the barrio and look for the plethora of funky restaurants, taquarias and cantinas. After dark, some of these places can be a little wild, so caveat emptor. We have a thriving Chinatown on the west side of town, where you'll see miles of streets with bilingual street signs in English, Chinese, Korean and Vietnamese. You can find just about every kind of Asian and Southeast Asian food you could want there, that ranges from the amazingly good and authentic to the truly terrible and authentic.
As for space stuff, JSC is really great. Unfortunately, Space Center Houston is not as satisfying for an adult as you'll expect. To really appreciate JSC, you need to take the tram tours into the actual facility. If you want to get the most out of it, you need to devote the whole day so you can do both of the tram tours -- the one that will take you into the main campus (including ISS and STS MCs) and the off-campus trip to the Neutral Buoyancy Lab. In between, ignore the kids stuff at Space center Houston and concentrate on the Skylab mockup and walk-through Mercury-to-Apollo displays, and take in as many of the films shown in the theater as you can.
Hope this helps.