The observation deck on top of the Stratosphere Tower is worth a visit. Not free, but there may be a discount packet for it when you sign up for their slot card. If you have a little time, go about 1/2 hour before dark, then stay to watch the sunset and the LV Strip lights come on.

Also:
Caesars Palace - Forum shops, including 2 anamatronic statue "performances". Free

Aladdin - Desert Passage shopping area. Free

Mirage - White Tigers, across from the gift shop. Free (There's also a dolphin/tigers/elephants, etc. exhibit, but there's an admission fee for it.)

Monorail from MGM Grand to Bally's. Free
Also, elevated tram rides: Excalibur > Luxor > Mandalay Bay. Also Mirage > Treasure Island. Also Monte Carlo > Bellagio. All Free

Imperial Palace - antique auto museum, IF you're into autos. Otherwise, optional. Look for free coupon for admission in hotel magazines or in a funbook.

Of course, the themed architecture of the newer properties is pretty amazing. Especially Paris, New York New York, Venetian. But most the post-Mirage properties are well worth seeing just for the theming.