". . and looking to stay a little south of downtown. . "
Not real sure why that (little south of downtown) is your preference but if you want to be near the light rail line I agree with houstoner
. The rail line runs through the Museum District and the Texas Medical Center. There are numerous hotels in that area. That area is basically southwest of downtown, 3-4 miles but 10-15 minutes by rail.
I found several using orbitz.com, Houston hotels
, and click on "Museum District, Medical Center, Reliant Park
". Several should meet your $100/night requirement.
Also, Google Earth (FREE at More Google Products
) shows numerous hotels in and near the downtown area.