I live nearby and this is one of my favorite state parks for many reasons. There are so many areas to explore, camp and take photos at that you can find a new area every time you visit.
There are plenty of campsites and areas to 'picnic' at both near campsites and independent of them. There are a couple of small areas with a sandy 'beach' to make it easy to get into the water for kids.
For those wanting to fish or just relax without the splashing, the rock ledges along the water in several areas are the perfect spot. There are areas along the ledges that you can get in the water, but not too far which also allows you an area for a fire when allowed.
It's an extremely popular spot for photographers as well. You could take all of your clients there and almost never have to shoot in the same location....even though most do. Depending on the time of year, the grasses/flowers are gorgeous and the sunset over the lake is amazing.
Like most lakes in Texas, Grapevine Lake is no different. It will rise easily during storms and rainy season which will cause access to some areas impossible so make sure to check the website online first. They do also close areas for upgrading.