Most teams are much more open than the Texans are with regards to their training camps, so you'll have to hurry if you want tickets to gawk at the Texans during their sessions. Per Nick Scurfield's blog post on the mothership, the dates open will be the following:
↵↵↵Friday, July 30 - 8:30-10:30 a.m.
↵
Saturday, July 31 - 8:30-10:30 a.m.
Sunday, Aug. 1 - 8:30-10:30 a.m.
Monday, Aug. 2 - 8:30-10:30 a.m.
Thursday, Aug. 5 - 7:15-9:15 p.m. (only open night practice)
Saturday, Aug. 7 - 8:30-10:30 a.m.
They are first come first serve, limit 10 per practice per person, and you can nab them for free (zero dollars) right now at this link. I grabbed tickets to August 5th, myself.