I would plan on visiting late April thru early June. This is the busy time, but the waterfalls will be great.
Plan on staying at the TT Yosemite Lakes Park near Groveland, CA. $50-60 a night. Just a short drive to the NP's entrance. Then a beautiful drive into the Valley.
From the TT park, it is a nice trip to Hetch Hetchy Valley.
If you are doing an extended stay, consider buying a TT Zone pass. This will give you 30 nights free camping, then just $3 a night. At times, you can get 2 Zone Passes for the price of one.
PM me if you like.