4 fellow travellers? Rent a car (which is not much if split 5 ways). You can see a lot. Look up Houston on the internet. Virtualtourist.com has some amateur pages on many cities. Convention bureaus also sometimes have tips.

Some things to choose might include the I.M. Pei designed Chase Tower (formerly Texas Commerce Bank) which is the tallest gray building with the Miro sculpture outside, the Menil Collection (a free museum), Galleria (expensive shopping center), Astrodome (outside), funky Montrose neighborhood (not that funky), NASA, and I'm sure a lot more that Houston natives know about.