How much time do you have for your trip to the Finger Lakes? I would highly recommend Canandaigua Lake. It's the largest touristy Finger Lake near Rochester. Only about a 25 minute drive from Rochester. There's a little beach on the lake although I do think it costs a few dollars for access to the swimming area. The waterfront there is beautiful. Ignore the ugliness on the other side of Lakeshore Dr. though because there is a very large project (North Shore) starting up there. There's also a hidden neat little walking gem called "Lagoon Park" down near the lake too.
This is the walk I do frequently. Although walk through Kershaw Park where it's showing walking on the outside of it:
City Pier, Kershaw & Lagoon Park Walk in Canandaigua, NY, United States | MapMyRun
If you have time you can visit downtown Canandaigua too. There a lot of neat shops and it's a very historic downtown. Canandaigua is actually older than Buffalo and Rochester so it has some really cool history. Susan B. Anthony's trial for voting was conducted in Canandaigua. The person who got the Erie Canal off the ground devised his scheme for how he was going to get it started while in debtor prison in Canandaigua. And there's a beautiful mansion, garden museum called Sonnenberg Garden.
Sonnenberg Gardens - Wikipedia, the free encyclopedia
Also, Ontario Beach is very, very safe. Don't worry about safety down there. The few isolated issues have been blown way out of proportion. I've walked down Ontario Beach more times than I can ever count and have never had one single incident.