Mandalay Bay to Excalibur
This sleek train runs from the corner of The Strip and Tropicana next to Excalibur south to Mandalay Bay and then back again with a stop at Luxor along the way. Note that the southbound route does NOT have a stop at Luxor. It's free and runs about every 10 minutes from early in the morning until late at night.
Mirage to Treasure Island
Adjacent to the main entrance of The Mirage you'll find a tram that will deliver you to the rear of Treasure Island. Even though the hotels are right next to each other, this will probably save you time. The tram is free and runs about every five minutes during the day and evening.
Mandalay Bay to Luxor
There is a store-lined bridge (known as Mandalay Place) from Mandalay Bay to Luxor. Access it from Mandalay Bay near the showroom and House of Blues and from Luxor near the main casino cage.
Luxor to Excalibur
There is an indoor bridge with a moving "sidewalks" between these two hotels. Access it from Luxor near the high limit slot lounge and from Excalibur on the second floor attractions level.
Bally's to Paris
There is an indoor walkway with stores and restaurants between these two hotels. Access it from Bally's near the main casino cage and from Paris at the back of the shopping area just before you go up the escalators to the parking garage.
Wynn to Encore
There is an indoor walkway with stores and a big nightclub from Wynn Las Vegas to Encore. Access it from Wynn Las Vegas near the showrooms and from Encore on the opposite side of the casino from the reception area.
The Venetian to Palazzo
There is an indoor walkway with restaurants and stores between these two hotels. Access it from The Venetian at the end of "Restaurant Row" and from Palazzo off the main casino toward the Palazzo Shoppes.
CityCenter, Monte Carlo, and Bellagio
CityCenter has a monorail that connects it to two sister properties, Monte Carlo and Bellagio. The Bellagio station is at the far south end of the property, past the Spa Tower. A walkway connects Bellagio to Vdara at CityCenter but hop on the monorail and you can go to the Crystals shopping mall or on to Monte Carlo, where the station is located at the end of the Street of Dreams retail promenade. Note also that there is an enclosed walkway between Monte Carlo and Aria.