Houston is flat. Up north, there are some rolling hills. But you have to get up around Conroe to find that kind of terrain. The best you can do in Houston are banks of the bayous and terrain created by roadways. They are all going to be short hills, but can be effective for training.
Here are some of the favorites:
1. Memorial Cloverleaf: There are four very moderate hills on the cloverleaf at Memorial Drive and Waugh Drive. Main benefit is that you can run up and down all four without having to turnaround.
2. TC Jester bridge:
http://www.gmap-pedometer.com/?r=6192028
Good climb for about 200 m up an overpass. Only problem is that it is all concrete and hard on the shins.
3. Buffalo Bayou between Shep and Sabine: This is a moving target as they are doing a lot of construction on the bayou. But there are a number of spots on the banks that are good for short hill repeats.
4. I-45 and Heights Bike Trail:
http://www.gmap-pedometer.com/?r=6192041
About 100m on a good grade up the bank of the bayou from the bike path (between the new community center and the house with the garden to the southeast). I like this one because you can run on the bike path and finish a 400-800m interval by climbing the hill. And there are rarely other runners on it.
5. Entrance to Bayou Bend (Westcott at Memorial). Longer hill up the entrance to Bayou Bend.
6. Hoggs Woods in Memorial park:
http://www.tkvw.com/mpc/images/memorial-park-map.pdf
Some very short little hills on dirt trails in the park in a small section that is supposed to be for runners only (no mountain bikes). Accessible by some creepy tunnels under Memorial Dr. or Woodway. Fun place to do some trail running, but also a good place to twist an ankle.