When are you arriving?
I've never been to Tahoe, I'm not sure why, but have spent a lot of time in the high Sierras.
I would rent a boat and cruise or fish (drowning worms) or both.
You can rent mountain bikes and use the ski lifts to take you to the top of the mountains and ride the bikes down the trails. Then, repeat.
Go for a day hike. Bring a day pack, food, WATER! and warm clothes. Also, bring WATER! and more water, too.
There will be so much to do.
A few words of caution but don't let this scare you off, you'll have a great time.
Altitude sickness is a real threat and can be either inconvenient or a real threat. Even a mild case is simply no fun at all (crushing, debilitating headaches, nausea). This can kill the trip for those affected. Tahoe is relatively low (6,000 ft) but is no joke for those not used to it and everything goes up from there (8,000 - 10,000 - 12,000 ft is not uncommon).
Dehydration is also a real threat and couples into altitude sickness. Drink, drink, drink, if you are peeing anything other than clear, drink more. Drink as you are driving there to get hydrated and stay hydrated, even though you will have to stop for frequent restroom breaks. It could save your trip.
ETA - Sunscreen and Chapstick