GRSP is the first park Justin and I ever camped at together (during our dating years) and we live near it these days so it's been a staple when we need to get out among nature. It's been awhile since I've been out there due to pregnancy and now caring for our 2 month old, but here are some trip reports from our archives --
Hiking the Guadalupe River Multi-Use Trail (we have since mountain biked it on many occasions, as well.
Celebrating the Start of Summer one year at the park
Guadalupe River Camping
Way Back When ...
Needless to say we have spent a lot of time out there. It's a family friendly park with a nice trail system, river access, good picnicking and camping, etc. We prefer the walk-in tent camping area but the drive-to spots (w/electricity, etc.) are good as well. We've basically had sites in ALL areas of the park except the Boy Scout camp.
Highly recommend taking the Honey Creek Guided Hike if it's offered during your visit, btw.