Winter 

1. Go Skiing - with 9 different ski resorts around Lake Tahoe, there's something for everyone.  Squaw and Heavenly are favorites for everyone and those looking for more extreme skiing will enjoy Alpine Meadows, Sugar Bowl and Kirwood.  Homewood and Sierra Tahoe are local favorites and a little more private where you can often still get powder a few days after the storm.
   
   
2. Snowshoeing - Pick a spot and go!  Lots of places to rent snowshoes at.  Just be Avalanche weary, especially if you're gonna drag a snowboard with you.
   
   
3. Ice Skating - There are 4 places to choose from.  The Tahoe Recreation Center in South Lake Tahoe offers a full indoor rink at reasonable prices.  The Downtown Marriot offers an expensive but cute little rink right downtown, or you can go play at the Squaw Valley rink on the mountain top, or the ground level one at Kirwood.
   
   
4. Boat Cruise on the MS Dixie - Always fun, this cruise goes all year round.  Winters are so mild here, you can enjoy a hot bailey's and hot chocolate while breathing in the fresh air and soaking in the spectacular Tahoe views.

Summer

1. Emerald Bay - Lots of options to see Emerald Bay.  The first is by foot, park at the top and walk down. Second is by boat, you can rent one, or take the Tahoe Thunder from Timber Cove.  It's the fastest boat on the lake and a really fun cruise.  You can also jump on one of the big paddlewheelers: either the MS Dixie or the Tahoe Queen will get you there.
   
   
2. Waterfalls - Some of my favorites include Eagle Falls (upper or lower), right next to emerald bay, hike up for upper, down for lower....  Or you can see the Fallen Leaf falls, by driving past Fallen Leaf lake, keeping straight at the firestation till you see a lot of cars parked by the dirt road.
   
   
3. Hiking - Pretty much everywhere is good.
4. Mountain Biking - There are good lists around the internet.
5. Lunch on the Lake - Some favorites include Riva Grill, The Blue Water Bistro, The Beacon and Sunnyside.
6. Beaches - Zephyr Cove Beach is a lot of fun (waitresses serve alcohol on the beach), Timber Cove Beach is nice and close and quiet with a bar/dock next door, Nevada Beach allows cans of beer and provides bbq pits, lots of privacy and a huge beach.
7. Kayaking/Jet Skiing/Boating - Once again, there's rentals pretty much everywhere around the lake.