If staying over this summer, suggest 2 adventures w/ no car needed:
If fishing, (license needed), from the Village area (a free bus nexus), take the free Twin Lakes bus all the way past Twin Lakes to Lake George - good stocker fishing NE side, also could fish the exit of Mamie, (BTW, return bus comes every half hour summertime)...or fish the waterfall area of Upper Twin, or...or screw fishing - take the bus up and walk down...multiple lakes, maybe visit the old mines?
or pack a lunch and take the same bus to Horseshoe Lake (above Mary). Hike up to McLeod Lake (easy) and then over Mammoth Pass and about 1.5 hour hike down a scenic trail to Reds Meadow Resort...Old time milkshakes
the rest of the menu 
- (why?...too expensive for words). Down at Reds another free bus takes you on a low loop - easy hikes to see Rainbow Falls - Devil's Postpile - Sotcher Lake - Agnew Meadows and back. A $7 pp bus from DPP takes you back up the hill to the Main Ski Lodge - and another free bus there meets / takes you down again to the Village - where another free bus also connects you to Vons and the rest of Mammoth.