There are a number of free things to do in town, many of them excellent! I'm going to cover my favorites here. If I've left off one of your favorites, send me an e-mail and I'll include it with your comments.
My personal favorite free thing to do right now is the Fountain Show at Bellagio. The schedule was confusing when we were there in December, but it was basically every hour on the hour from dusk to 7pm. Then every half hour until 10pm and again every hour until midnight. I would check with the Bellagio to be sure of the schedule so you don't miss it.
This show is not like any other show in town. The fountains are HUGE and the sound system is excellent considering the entire event takes place right on Las Vegas Boulevard. When we were there in December, the shows were all set to Christmas music, but others have reported music like "Luck Be a Lady", etc. I have pictures from my trip here.
While you're in the neighborhood, you should walk next door and check out the The Mirage Volcano - A Las Vegas Icon Reborn. After dark, the fountains in front of the Mirage burst into a violent, volcanic eruption complete with flames, rumbling noises and searing heat. Get up close on the sidewalk and you can almost burn your eyelashes from the heat of the flames. It's a lot of fun and not too difficult to get to or see.
I used to recommend people who want to watch out of the crowds go across the street and watch from the sidewalk next to Denny's, but with Venetian's opening, that area has become rather crowded. However, I did enjoy watching the volcano from the front entrance area of Venetian, but there's nothing quite like being right up front to feel the heat!
If you have a car, or don't mind paying some taxi fare, I've got some more recommendations for you that are off the strip.
Fremont Street Experience - Even if you don't have a car and you don't have a big budget for taxi rides, I'd recommend you spend the $10-12 taxi fare or spring $1.50 each for the bus ride to downtown to check out the Fremont Street Experience. Every night Fremont Street has light shows on the overhead canopy each hour. I love the shows and never get tired of them. Other people say that once you've seen one, you've seen them all. However, the one thing people agree on is that you've got to see this!
The shows start about 8pm (times vary depending on the season) and go every hour, on the hour. My absolute favorite part of the show is the one second before it starts when the sound comes on, the "conductor" taps his baton and all the lights go out on Fremont Street. Darkness in Las Vegas isn't something you see often. After a brief moment, the show begins and you're surrounded in lights and music. I love watching people here. I am always amused by conservative looking people singing their hearts out along with Wayne Newton in the Viva Las Vegas theme show, or people doing the two-step during the Country Western themed show.
I suggest you get downtown, and see one of the shows. Then check out some of the smaller, downtown casinos. Or go down to Main Street Station and have a beer in their Microbrewery (the Marker Pale Ale is our favorite). Or if you're with kids, walk down towards the Fitzgerald's end of Fremont Street (away from the Plaza) to see the Neon Museum. There are a number of gift-kiosks where you can buy t-shirts and other oddities. Before you know it, an hour will have passed and you can catch a second show. Schedules, information and facts can be found at the Fremont Street Experience web site.
Parade In the Sky at Rio Suites is really fun. Dan rode in the parade on our trip in December. The pictures are here. To ride in the parade, you need to either be a hotel guest (like we were) or go to the ticket booth and pay a mere $9.95. Dan described the experience in our December Trip Report (December 24th).
Basically the parade is a series of floats that are suspended from the ceiling above the casino in the Masquerade Tower. They are even with the second floor shops. There are several different themed shows and each one is full of energy and great music. Dancers and performers throw strands of beads as well as the occasional buffet "2 for 1" coupon into the crowd. It's fun! The show takes place every day except for Wednesday, every other hour from 2pm until midnight (2pm, 4pm, 6pm, etc.). Information and pictures are here.
Sunset Stampede at Sam's Town - This last one is WAY out of town but makes for an enjoyable afternoon/evening if you have a car. Sam's Town is out past Boulder Station on the Boulder Highway. The Sunset Stampede is their laser/water show. It runs every other hour from 2pm until 10pm. If you get there early, grab yourself a seat at the Ram's Head Bar, get a drink and relax in the atrium. It's beautiful and I find it very relaxing. If you're not in the mood for a drink, go to their ice cream parlor. It's the best ice cream I've had in Las Vegas.
This is a locals place. There are a number of excellent gaming opportunities including full pay Video Poker (I hit my first Royal Flush here!) and low stakes Blackjack. I enjoyed a good casual dinner upstairs at Mary's Diner and I hear the other restaurants are equally good. I had really wanted to try to Mexican restaurant there, but our time was limited and it wasn't open when we were hungry. We plan on visiting on our next trip. Pictures of the show can be seen on our site here, or you can go to Sam's Town's web site here.