The Downtown Loop is a free shuttle bus with stops around downtown Las Vegas and the Arts District. The closest stop to the Plaza is across Main Street and one block north at Ogden. Hours are 10a – 3p Sunday – Thursday, and 3p – 10p Friday and Saturday.
The Deuce is a double-decker bus running the length of the Las Vegas Strip, with a stop at Fremont Street and Las Vegas Blvd (about 3 blocks east of the Plaza). The Deuce runs 24/7 approximately every 15 mins. Tickets are time-based, hop-on-hop-off, and are sold in 2-hour, 24-hour, and 3-day lengths, at $6/$8/$20, with a 50% discount for veterans, seniors, and people with disabilities. A ticket vending machine is available at the Fremont St/LV Blvd stop.
Other bus information can be found on the RTC website.
Uber and Lyft are both active in Las Vegas, and there is a designated pickup spot near the main entrance of the Plaza on Main St. Taxicabs in Las Vegas can be called to arrange pickup, but almost every hotel, mall, and other major attraction generally has a taxi stand where you just get in line and will guickly get a cab.
Airport Shuttles are available and can be booked in advance. This page on Vegas.com has a lot of good information about airport shuttles.