Seems like everyone else has SF covered, so I'll take Tahoe. (We lived up at South Shore for several years)
Try Fire and Ice for dinner one night. It's in heavenly village, and a pretty cool place. It's kind of like upscale Mongolian grill, where you pick which meats, veggies, etc that you would like, all cooked up on a big grill in the middle. There are a wide variety of sauces to pick from, so you can get a lot of different flavors.
For waterfront dining, your biggest choices are Riva Grill, Fresh Ketch, and the Beacon. Riva is the fanciest of the three, followed by the Beacon and then Fresh Ketch. Fresh Ketch is on the water in the keys, so the view is not as impressive. The Beacon has fantastic fish and chips and calamari, as well as Rum Runners if you enjoy a nice alcoholic beverage from time to time. I would eat there for lunch, rather than dinner, as I feel dinner is a bit overpriced.
For casual food, my top two choices are Burger Lounge or The Brewery. Burger Lounge has fantastic burgers, fries, and shakes (try the Nutella shake!) and was featured on Rachel Ray's show some years back. It's a tiny little shack almost on highway 89 pats the Y. The Brewery is on the main drag, and has pretty good food, a nice little atmosphere (almost feels like a house with a fireplace and all) as well as good beer.
What are your plans for Tahoe? Skiing/snowboarding? On your way into town, while you are coming over the mountain, you will pass Sierra At Tahoe. They have a fantastic snow and tubing park for kids.
If there's anything else you'd like to know about the area- let me know!
Oh, and as to the rental car part of it- rent an SUV. Seriously. It's just easier. They say we're in for a heavy winter (lord knows we need it) and going over the summit is just easier with 4WD. Not to mention the icy roads in town.