Hi!
My girls are exactly your kids ages (2 & 4 yrs) and this is what I usually do with them reguarly:
*playdates with other kids thier age. Usually 1x a week
* Free Family Movie Film Fest at Regal Theaters every Tuesday and Wendsday morning at 10am. This is all summer long, and a pretty sweet deal being toally FREE. Not every movie they show is appropriate for a 2 and 4 yr old - but they give you a choice of 2 movies each week. Last week was Shrek3 and rugrats. We saw Shrek 3, and that kept the girls pretty happy to come along with me grocery shopping afterwards.
*reading times, story times etc at the local Libraries. rthere are LOTS of activities at the local libraries here in town. It;s awesome. If there isn;t a time and age group convienant at your nearest Library, just check out another.
You don;t have to have a lirary membership to take part, although the membership is free and I'm sure you'll end up with one. My girls are now book/library addicted. :)]
* the Witte Museum ha free admission on Tuesdays. which is great for the huger HEB treehouse in the back which is filled with all kinds of hands on science stuff. My 2 girtls LOVE it. Also right behind the Witte in breckenridge park is the duck pnd, pavillion, a great playscape area, huge trees etc etc. You can totally make a nioce , kid friendly relaxing day of it :)
Now this next suggestion is not *free* but it works out to be really cheap in the long run, so I always suggest it to everyone I know.
*get a Zoo Membership. There are free group activities in the Tot Spot everyday. Plus they can swim in the riverbank at the Tot Spot as well. When you ahvethe membership it is no big deal to just go for an hour or 2 hours at a time, you don;t have the pressure to be there all day and see everything. Some days we go just for the animal encounters, and to play in the sand on the riverbank and cool off :)
The family membership is $60 total for your household. And after you;ve gone 2x's it;s really paid for itself as the regular admission would be $9 for parents and $7 for your 4 yr old.