The Pinball Hall of Fame (
www.pinballhall.org): Free admission, but playing the machines costs money obviously. You can stay entertained here for hours on a few bucks in quarters though. It's a couple miles off the strip on Tropicana, so if you're staying on the south end of the strip (MGM, Luxor, Mandalay Bay, Tropicana, etc.) it's walkable if you're adventurous and the sun's still up (not the greatest neighbohood). If you're center/north strip or downtown you can get there by bus easy enough.
The National Atomic Testing Museum (
www.nationalatomictestingmuseum.org): $14 bucks admission, but well worth it IMO if you're into military tech and/or cold war history. Easily walkable from center strip or via bus if you're coming from farther away.
Hiking at Red Rock Canyon: You'll need a car for this one. Absolutely beautiful out there though.
Fremont street/downtown: The light show on Fremont is worth checking out and costs nothing.
CET "Buffet of Buffets" (
www.totalrewards.com/microsites/las-vegas/buffet-of-buffets): $50 for a 24 hour buffet pass to all the CET casinos. Time it right and you can get 2 dinners a breakfast and a lunch. $12.50/meal/person isn't exactly mustachian, but you will not be able to beat it on the strip unless you go strictly fast food.
Walk from casino to casino, stop in at each one, sign up for their player's club cards and see if they're running any promos for new signups. You should be able to grind out a few bucks worth of free play and/or some discount meals. Cosmopolitan has been running a $100 slot loss rebate that is a very good deal. Basically, if you have a net loss of $100 in a calendar day, you can take your card to the membership desk and they'll load $100 of free play on it. Then you can run that $100 through a lower-variance machine (video poker works well for this) and expect to get ~$90 of it back. The killer part about this promo is that the $100 resets every day. Meaning you can go in and sign up on your first day, then plunk your $100 into a machine and start playing. If you lose the $100, go get your freeplay. But if you win, cash out and come back the next day. The required loss for the promo resets at midnight. You could in theory win a million bucks on Friday night, then come back on Saturday, lose $100 and claim your $100 rebate, even though at that point you'd be lifetime ahead $999,900. In practice you will come out a few tens of dollars ahead or behind the vast majority of the time, but it reduces your risk of loss considerably.