A large group of friends from college (a couple dozen) gathered in Sun Valley for Memorial Day weekend festivities in a resort setting. The weather was fantastic, though it's certainly not guaranteed to be summerish at this time of year.
We stayed in condos in Elkhorn and spent our nights eating, drinking, and hanging out there, but we did manage to fit in some fun activites in the area as well: Golf at Bigwood Golf Course, a 9-hole course in Ketchum. This public course was best suited to our group (more hackers than not) -- didn't hurt that it cost less than any other option in the area, and the course layout is interesting enough to satisfy good golfers as well. Fox Creek loop hike. The Fox Creek trailhead is just a few minutes from the north end of Ketchum. It's an easy 5.5 mile hike with some nice views, also used by mountain bikers.
We had about 10 people on the hike and it worked well in terms of everyone being able to keep pace. Scenic drive to Galena Summit. A handful of us drove north 30 miles to the Galena Summit and its beautiful view of the Sawtooth Mountains (yup, that's what they look like). The entire drive was quite scenic, and there were a number of places to stop and take a walk, view the river, enjoy the mountain views, etc. There's even a mountain goat viewpoint, though the sightings are anything but frequent.
Biking - there's an extensive paved trail system in and around Sun Valley, Ketchum, and the Wood River Valley generally. Swimming - one nice thing about staying in Elkhorn was the convenience of the main Elkhorn pool (near the Elkhorn golf course). We also had enough guys to run a full-court outdoor pickup basketball game in Ketchum.