There are lots of courts sprinkled about, but you need to know where to look. If you're in town, in addition to Memorial Park, Homer Ford park east of downtown has a lot of courts and a solid tennis scene, with easier court availability than Memorial (but the courts aren't as nice). There are some other parks in EDO that I know of that have one or two courts (like Emancipation Park). West U has a few city parks that are hiding some tennis courts, as does the Heights (Woodland Park, Halbart Park, Proctor Park, etc.). Houston is such an odd city, though, that you need to do some looking to find them. You might want to get on Google maps, find your area, and do a search for "tennis." The suburbs have lots of courts as well, sometimes in parks and sometimes as part of subdivisions that have restricted access. Aside from Memorial and Homer Ford, I think a lot of the tennis scene goes to the private clubs.