Just returned from Moab two weeks ago on a mountain biking trip. Great place with good trails for you and your wife.

First stop would be the Moab Brands area, just north of town on the way to Canyonlands NP. You can get trail maps at all bike stores in town or the maps are posted at the site. Easy flat trails like Bar M, a little more challenging Lazy and EZ, and more challenging North 40, Mavericks etc... If you want a taste of slickrock try Circle O or Rockin A (more challenging though). Opinion was that Circle O beats the crap out of you and is more rough than the Slickrock Trail in the Sand Flats area.

Up Canyonlands NP area is Navajo Rocks with a few more challenging trails, in the sense of distance(each being 1-4 miles long), or further into Dead Horse Point Park where they have the Intrepid Trail system. That is a perfect beginners area. At Intrepid you can do easy flat single track or more scenic loops like the Pyramid Lookout Trail. You can easily walk around any hard sections. Mostly moderate up and down single track with the occasional drop or ledgey bit.

Slickrock Trail itself is considered a bit over rated but I enjoyed it. It's main draw back is length and steep climbs but you can walk those if you want. Try the practice loop of about 2 miles as it is what you get for ten. The main path is easy to follow and I started early and had most of the desert to myself.

My advice would be to hook up with a shop for advice. I went through Poison Spider and Amy and Tim were great. They'll recommend trails and provide maps etc... For beginners you can spend a lot of time at Brands and Intrepid working on your skills and confidence and then try more challenging stuff. The scenery is stunning as well and I would suggest Touring both Arches and Canyonlands/Dead Horse Point Parks as well as driving the Colorado River canyon (Hwy 128).

I'm at work right now but will link my slideshow when I get home.