Yosemite Falls is a great hike and you don't have to do the entire thing. It takes about 6 hours to hike to the top and back down so getting an early start before the heat of mid-day is good. Going up takes at least 3 hours but it's much quicker coming down and there are spectacular views along the way. Bring cameras! There is a swimming area at the top but be advised that it is slippery in places, and not to ignore warning signs about staying away from the edge at the top of the falls.
Bring water bottles or use something like a Camelback to keep hydrated. There are many water fountains throughout the park so you don't have to keep buying bottled water and it lessens the environmental impact of disposing the bottles. In the warmth of June days, chocolate tends to melt pretty quickly and messily - trail mix and/or energy bars might be better to bring. Trader Joe's has many different and tasty trail mixes.
A fun thing to consider would be renting a couple of rubber rafts at Curry Village and paddling down the Merced River a ways. At the end, there are shuttle buses that return you and the rafts back to Curry Village.
Even though it'll be the beginning of Summer, some of the higher-elevation places like Glacier Point are up to ten degrees colder than the Valley floor so plan accordingly. You'll have a great time in Yosemite.