Easter weekend? I'm sad to say that I'm not sure you're going to find a site if you're beginning your search today.
Here are most of the
parks near Houston.
I suspect
Brazos Bend State Park will be booked for Easter weekend (it's a terrific park, with numerous campsites), and I haven't personally camped at many other Houston-area parks.
I'm sure some of our East Texas folk (residents and/or frequent visitors) can chime in with additional guidance/information.
Good luck!