Well there are a few choices.
My favorite is Gyro Beach off of Lakeshore/Pandosy. However, the only thing that makes it my favorite is the sand and the bod watch aspects. Keep heading towards all the wineries in KLO. Any local can give directions. Sand beach and large shallow and swim area. Mostly a place where the locals go as well as the beach bums and bunnies who wish to show off their bodies. There are play areas for chidren as well. But not close to anything, so if you want something to eat, either bring it yourself or be prepared to pay lots. But still the best sand. Volleyball nets available and grassy areas.
City Beach is smack downtown and where most of the tourists go because there is so much to do there. Sand beach again but not as nice as Gyro sand. Volleyball, track, skateboard park, kids area. There is something here for everyone.
Tons of beach access along Abbott Street.
Do not swim in Duck Lake. It's called Duck Lake for a reason and what do ducks leave behind. Too shallow for anything to cycle out well.
Wood Lake is good if you want to drive all the way to Oyama which most of us don't but it sure doesn't get the crowds that Gyro and City Beaches do.
There are a couple other places on the Westside of the lake but it is so not worth crossing the bridge in the summer that I've never bothered to go.
Favorite thing: Kelowna is most notable for its incredible views of Okanagan Lake and year-round recreational pursuits. With a really nice beachfront park and plenty of marinas, there's no problem finding watersports. Boating, camping, and hiking dominate the summer month activities. Also, Kelowna produces wines that have a worldwide reputation and there are many vineyards that can be visited. During the winter, skiing at the nearby Big White Ski Resort is popular. Basically, this is where people who live in the Vancouver area go to get away from it all.
The Okanagan region is well-known for its orchards. If you like fresh fruit and vegetable and lots of them, you've come to the right place. Many large orchards and farms are off the beaten path, but many are right on the road, and there are many signs to beckon you. We found a very nice orchards just by driving around. Some also specialize in organically grown produce, so ask if it's your cup of tea.
Same goes for vineyards. Many offer tours and free taste tests.