We are going at the end of May to Venice and were wondering if there are any close beaches in Venice that are amazing for a few hours of relaxation?
Yes. Google Venice Lido for tons of info.
Venezia Lido is the closest. For a few hours it's closer than anything else. But don't expect the beach to be anything like Fiji or even Florida. Also the weather in May might not be so hot to require to dive into the water.
A better beach would be at Jesolo (Lido di Jesolo), but that would take you a little longer (maybe one hour) to get there and I'm not sure if it's worth it to take time away from Venice.
We went to the Venice Lido in 2011 in late May, and it wasn't really open yet. Yes, you can walk along the sand, but the concession stands, etc. weren't open. And with good reason. It was pretty gray. I would imagine that it would never be "amazing" in May.
I've posted a few photos here, if you're interested: Venice Lido in May 2011
The beaches in that area are far from amazing. Sand is like mud and water isn't clear.
I agree ... I was pretty underwhelmed by The Lido ... only did it to stick my toes in another ocean. If you must, go, but don't expect amazing. It's kind of missing the point I think to lose time in Venice to go here.