Haven't tried Costco but always worth a look.
Having worked for a famous car rental company that rhymes with worstz (and quit because they were idiots) I recommend the following:
1. First see if you can get on any of the car transport sites for SF to Reno/Incline: people pay to have someone drive their vehicle. Chancy due to the car seat situation though. And there are sometimes shuttles from the Reno airport to Incline (to the Hyatt).
2. Check to see if you can figure out a rideshare kind of thing at least to Reno, as then the car rental situation for Incline will be easier.
3. Definitely check each local rental carrier for specials and promotions...and sometimes the cc companies include special coupons in your bill (Amex does, anyway). And check to see if the weekly or monthly option is cheaper...I forget the cutoff point but especially at the other company that charges for miles...if you're near the cutoff point the longer rental adds up to cheaper versus the over mileage charges. You can always return early: wortz doesn't fine for that.
4. Check online to see if being a "gold club member" will help you...each company has their own promo thing and each one varies, big time. But I've always found Priceline Kayak/online sites to trump them.
5. Depending upon what company you choose, make sure that you either have unlimited mileage or know you can make your trip happen in allotted mileage...some are pretty short numbers. I'd mapquest your route, but keep in mind that you are so going to want to drive around Tahoe. And probably do some hiking things in Tahoe and everything is pretty spread out in this area. For biking I highly recommend Blackwood Forest as it's a lovely paved road that goes back several miles to a stream with a few places for fishing/swimming.
6. Ask for a AAA discount and "do you have any other discounts?" and "is this your BEST price?" because the way it works is...if you're bringing your car in to get it fixed, they will rent you a car at a lower rate than if you are a tourist. Yes, I know it sounds insane, but it's true. At the "H" company, anyway. But even the other companies have some wiggle room with prices on a sliding scale and things like 'vendor' discount (you work for a company they do a lot of biz with, like Ford) carries a hefty discount.
7. Do NOT buy the gas option!!!!!!!!!!!! Ever! Fill up at Arco with cheapo regular. And fill up in the valley as Tahoe is 20-30 cents a gallon more than Carson City. Reno is about 15 cents a gallon more than Carson City.
8. Call the actual location you are supposed to pick up the vehicle at one day early (keeping in mind that most will now pick you up for free at various locations, even your home) and ask to speak to the manager and get a COMMITTMENT (for what it's worth, ha ha snicker) that your vehicle will be ready when you're scheduled pickup time is. Why? Because at the one place in Carson City you will regularly wait 4-5 hours past your scheduled pickup time for them to have your vehicle ready. Sometimes more. And sometimes it will NEVER be ready. Ever. (trust me on this) Since most companies have a free cancellation policy, most local people and professionals who regularly travel here book with both big companies and then whichever one actually has the damn car is the one that they go with.
9. Be careful about "protection" which is their insanely overpriced insurance. Collision on a busy holiday weekend might be worth it as it's walkaway coverage. But all the other bs stuff isn't...one is for 'some' of your luggage...and ends up costing you 14 bucks and covers something like $300 worth of 'some' luggage with lots of exemptions...so basically useless. There are many package options and they gloss over it as though you "have" to purchase the big pricey one: you don't. You're not obligated to buy any of it. Due to drunks on the holiday it's your call but check with your insurance agent prior to leaving to see what they cover...supposedly as of 3 months ago, Visa was the best card to book on as they waived a lot of crap as covered in the event of an accident. But this crap changes all of the time.
10. Most important: try and pick the vehicle up in bright daylight or bring a flashlight and carefully inspect the windshield and the tire treads. You are responsible for ALL windshield damage and they're pricey...it can be difficult to notice a small star or one inch blip but they WILL charge you for a new windshield if you return with one. You change your own damn tires and the company I worked for regularly 'forgot' to check tread depth. They would also rent a car out with an idiot light on the dash saying "oh, it's for scheduled maintenance that's not due until you return the car" which is often utter bullshit: don't take any vehicle with a warning light, regardless.
Best of luck. Last time I rented a car I went online and got like a 47% discount.