On the edge of San Marcos, there is a Park named Five Mile Dam park. It is located next to recreational league soccer fields and contains the Blanco River. The park is one of the only ones In San Marcos where you can camp at. You can fish and swim at the top of the dam. I would strongly recommend not swimming past the damn due to the water being shallow and containing a strong current with many rapids.
I strongly recommend hiking at this park. There are plenty of trails for hiking. The ground around the river is all rocks, which provides an awesome terrain for hiking and giving you plenty of stability. Also due to the terrain, it would suit you best to wear close-toed shoes. The soccer fields, unlike the rest of the park would require you to rent it out. You can find information for renting out the soccer field by clicking the following link.
http://smayso.gotsport.com/clubsite/?p=9028
Five Mile Dam is also one of the best places to fish. There are a variety of different fish with all different skill requirements to catch. There are catfish which is not too difficult to catch and can be caught on virtually any lure or live bait. There are plenty of decent size and bigger bass for the more experienced fishermen. To find out more about the park click the following link.
https://www.sanmarcostx.gov/Facilities/Facility/Details/Five-Mile-Dam-Park-38
Hill Country Hiking 