7 mile beach was the nicest beach I've ever been to, by far (and I've been to several...).
I don't know how long the Stingray City excursion takes, but 7-mile beach can be done on your own time, assuming you have a couple of hours or whatever.
I don't remember the specific details, but we just got in some taxi or bus near the port, went a ways down the beach and got out at some random point - no resort or anything special. We found a spot on the beach, and at some point a couple of local guys came by who had chairs and umbrellas they offered to set up for us for a not-outrageous price. They also had a cooler of drinks and box of snacks they'd sell. I got the impression they were far from the only ones doing this - lots of people up and down the beach had inflatables out in the water you could pay to play on, also, and I think that different guys had different sections of the beach that they handled with chairs and umbrellas. And, it wouldn't have been too far a walk from our spot on the beach (we wanted a less crowded area) to a bar/restaurant. It was perfect for our family, and very low-stress. When we were ready to go back, we didn't have any trouble getting a ride back, either (but I worried more there, since there wasn't a stand or anything nearby). Had we gone down to one of the bars (with much bigger crowds) I think we could have gotten a taxi even more easily.