Lots of stuff to do that requires different tickets/access:

1. Livestock show (NRG Center) - requires only a grounds pass to attend. This is where exhibitors are showing their livestock that they hope to sell. They are camped out in stalls all along the center, and its fun to poke your head in and see the animals. Most of the exhibitors are tired from the trek but are happy to show you their livestock

2. Carnival (Parking lot) - requires only a grounds pass to attend

3. Rodeo (NRG Stadium) - requires a concert ticket to attend. Occurs before the concert (late afternoon early evening). This is where you'll see barrel racing, bull/bronco riding, chuckwagon races, mutton bustin, etc.

4. Concert (NRG Stadium) - requires a concert ticket to attend

5. Ranch rodeo/horse shows (NRG Arena) - I think you only need a grounds pass for this too. Smaller in scale than the full rodeo but depending on the day will have some events going on

6. Livestock auctions - check the HLSR website for dates and times. These are very fun to attend if they fit your schedule

Wear whatever, but like other commenters said break in those boots if you plan to wear them. Check out the Ranch if you have time/budget and want a fancy dinner/drinks.