We took the kids up to Donner Lake from Reno last weekend and it was a blast. We had lunch in Truckee and then drove a smidge more to Donner Lake, visited the visitor's center (I'm such a sucker!) and headed on a short 1/2 mile trail walk that our 2 and 4yo championed all the way to the boat stop. I highly recommend you fork out the dough for a canoe rental. It's such a fun way to explore.
In Numbers: Parking at the Donner Pass Visitor's Center is $10 but you can park outside and walk in. The center itself is free. The trail and Donner Lake are also free, so definitely go for a hike! If adventurous, bring swimsuits and jump in! $30 gets you a canoe ride for 4 for an hour.
For Kids: It's water and nature and there's just so much to see and touch. My kids loved walking along the fallen logs and poking at things in the water. They loved most the canoe ride though and my adventurous 4yo even jumped in!
For Adults: Nature that is relatively clean and kept and free.
Hikers, mountain bikers, equestrians, and photographers are loving this new trail; it will eventually be 23 miles long, encircling the lake. The first section, Tahoe Donner to Summit Lake, is finished. Access the trail at Donner Memorial State Park, or take the Boreal exit off I-80 at Donner Summit and go east a 0.25 mile to the Pacific Crest Trail parking lot. The trail is still in development and there are more trailheads.