Todd, Chilcoot is right, no 5-day passes. There are 72hr passes, though; $15 for all routes except Deuce and SDX (both downtown-to-strip routes) and $20 to include those two routes. You have to buy the passes at ticket vending machines, which are at most major stops.
Chilcoot is right, the WAX is a great way to get from either the airport or from Las Vegas and Tropicana to downtown--it's an express, takes the Interstate, $2 for a single ride or $3 for a 2hr pass or $5 for a 24hr pass (all available on the bus), or $15 for the 72hr pass as above.
Where will you be coming from when you go downtown?