Max, if you were pretty close to the square then you have probably already seen a bunch of what there is to see. Granbury is mostly B-n-Bs and antique shops and Texas tourist shops, or else fishing. Lots of retired people in the vicinity. There is a W-Mart (if you have to), but I don't think there is much in the way of a hobby shop, or even a craft store that might carry any rocket stuff.
Still, it is kind of fun to walk around the square and kill a couple hours poking around the shops. You could impress your girlfriend by showing her that you don't have be launching rockets ALL the time!
Weather here has been chilly, wet, and violent this past weekend but is supposed to be 80s and sunny for the next week. It is usually quite gusty/windy around N. Texas and you may have trouble finding a field big enough to ensure recovery, if it is safe to launch at all.
I have not been to the Granbury city park myself (on west end of Bus. 377) but I am told it has some baseball fields and open space that might do for a launch. I don't know though, when I looked it up on a map it is only one mile away from their little airport and it might be a bad idea to launch your 4xD-powered moon lander. You might have to look around some other part of town.
I don't know that I recommend the following, because it's kinda tacky to go visit someone's parents and then drive off for the day, but if you want to go somewhere there is the Dinosaur Valley State Park (the place where you can see the fossilized dinosaur footprints in the river) and the Fossil Rim Animal Park (you can drive through and see a bunch of 'wild' animals), both are a couple miles (4-5?) west of Glen Rose, which itself is 1/2 hour south of Granbury.
Good restuarants in the area include Pasta Fina (italian; about a mile south of the square on S. main st.), Don Ayala's (mexican; east of the south end of the lake), and a lot of others I have no idea about. I know there is a Chili's on Hwy 377 right before you get to town, and there is the usual assortment of fast food holes.
If you absolutely MUST have some rocket stuff, the closest hobby shop with anything of interest is in S.W. Fort Worth. Olde Time Hobbies is just off S. Loop 820/I-20, take the Trail Lake exit and go one block south, it will be on your left at the back of the shopping center (go behind the Sonic drive-in). I think their store is labeled 'Toy Shoppe' or something like that. But I warn you: the girlfriend (and especially her parents) will not think it's cool for you to spend two hours driving in to Fort Worth chasing rocket stuff.