Brenham actually has a geo tour that's designed to make a day or a weekend out of it.
As of right now, I believe they are the only actual geotour in the state.
http://www.visitbrenhamtexas.com/geocaching/
There are also a series in Spring along Spring Creek - Greenway Trail Hike or Bike that looks like it could be a day's worth.
What are you looking for - park and grab skirt lifters or larger caches with swag and a bit more challenge.
Brazos Bend State Park is also supposed to have a pretty good series.
We did a series called "Getting the Goose" that stretched from North Austin to almost Bastrop. It's been a few years though. Don't know if the series is still there.