If you're in Vegas, a visit to the Pinball Hall of Fame is worth the cost of renting a car to get there. I'm no pinball wizard, but I love to play and anyone mildly interested in it could easily kill a few hours here.
Take a walk around before you start playing - this is easily the most impressive collection of working pinball machines under one roof. The H of F houses everything from the original KISS machine to Evil Knievel to Elton John's "Captain Fantastic" and (my personal favorite) the PUNK pinball machine. Try your hand at playing one of the relics and then move on to some more recent machines (Pirates of the Caribbean and Family Guy) - the older machines are $1.25 for 5 balls and some of the recent machines are a bit more.
Either way, you'll have more fun pumping quarters into these machines and $20 will last longer here than at Slots of Fun.