We enjoyed everything about Grand Isle and only wish it hadn't rained. Every. Day!
Oh well, our grassy site (#80) was a fine refuge for three nights in a tent. It was level, free of roots and stones and far enough/close enough to the bathroom.
Bring quarters for the showers (and pay it forward).
Bring your bike - lots of safe, local (flat) roads both on and off the campsite. Make sure you explore all the islands. (There are two nearby breweries.)
There's a so-called beach on Lake Champlain, but you probably wouldn't spend much time there. I like that they have a Dog Beach.
There are lots of signs warning against late-night noise. But the only thing I heard was a couple coyotes (I think).
The park is huge with tons of space for picnics, beachgoing and - weirdly - parking. (Someone must've gotten a tasty asphalt contract.)
Unfortunately, all of the campsites are jammed into one quadrant, with very little distance between each. Our site (#17) was quite small and was not clearly marked. It seemed to run into #18. We were told we could spread out into two sites since the adjoining site wasn't taken. (By the way: Site #19 looked to be the best in our neck of the woods.)
The roads to the tent sites are quite rocky and can be difficult to navigate.
The facilities are a bit warn, but the maintenance staff does a good job of upkeep. Since it was raining during our visit, it was impossible to keep the individual shower/lavatories clean for very long.
Though the camp was mostly filled, noise was not a problem during our stay.
The lake is quite nice and the swimming area is well-protected.