We typically don't use lifts on our mountain bike adventures, but let me tell you of a few places where there are trails you and your wife might enjoy:
You mentioned Buena Vista and there are some great trails there. For beginners, the portion of the Colorado trail on the south side of Twin Lakes (closer to Leadville) from the dam out to Interlaken is relatively level, just above the lake, and in the trees. We took our 11 and 13 y.o. nieces there last month and they loved it. There's also the paved
Mineral Belt Trail around Leadville. Not a singletrack, but the elevation makes up for that. Real interesting history panels to stop and read on the trail.
In BV, head east from the stop light to the park at the end of the road. Ride across the bridge and follow the signs for the Midland Bike trail (avoid taking the Whipple trail). The first mile or so is a little challenging in a few places, but we walk those parts. Eventually you get on an old railroad grade, a graded gravel road at first and then a great single track. There are places where the old railroad trestles are gone, so there's singletrack through bypassing those areas. We walk for short distances on the tough sections, but most is easy singletrack. We've also done this trail with the nieces, so it's simple, yet a place to get exposed to some easier singletrack and build skills. One can also head south to Nathrop and up Cottonwood Creek, past St. Elmo to Hancock. From Hancock, hop on your bikes and ride up the old RR grad to the east portal of the now collapsed Alpine Tunnel. Gentle grade at altitude. You can actually ride this grade all the way from St. Elmo for a longer ride on the graded gravel road. Interesting mining ruins along the route.
Salida is a very bike friendly town with a vibrant restaurant and burgeoning art scene. There are trails right out of downtown for all levels. Check out
this link for a brochure describing a lot of trails in and around the area.
While there's no lift serviced terrain in the area, for your downhill fix, have you wife shuttle you to the top of Monarch Pass and ride the Monarch Crest trail back to town (page 38 in the above brochure). While the first part of the rice is above timberline, giving your lungs and legs a workout, by the end of the ride, our hands are tired from all the braking on the downhill sections. There's nearly 4000' of vertical elevation drop on this ride!
We can easily spend a week riding in the Salida/BV/Leadville area.
Of course Durango has lots of singletrack of varying difficulty.
Link The Colorado Trail from the top of Molas Pass isn't too technically difficult, but the elevation will give your lungs a workout. The flowers were great there this spring, but by September, most will be done by then. The views make that ride worth the effort. From Durango, you can also head over toward Cortez, north to Dolores and then north to the
Boggy Draw Trailhead. We ride out Mavericks Loop to Italian Canyon Loop, back to Boggy Draw Loop. Great, flowing singletrack with some fast yet relatively easy sections to really get going.
Of course from Durango, it's not far to Telluride. Lots of great mountain biking there. The gondola from town is free for you and your bikes.
Map Your wife can stick on the blues while you're on the blacks. And it's free! The Riverside trail runs through town and both west and east. The line to the west (downvalley) is especially fun as an out and back. To the east, the trail has historical markers to stop and read. The whole trail is really a blast. Just west of Telluride you can hop on the Galloping Goose trail. The first part is on graded road, then singletrack, all up old RR grade. Up past Ophir Loop, you can continue on dirt/gravel road all the way around Trout Lake to the top of Lizard Head Pass.
While it may not be on your agenda this year, I would be remiss not to mention Fruita. We just got back from there and once again had a blast. The 18 Road (aka Bookcliffs) area has nothing but singletracks of a variety of difficulty. Prime Cut up and either Kessels or PBR back down is all straight forward and a blast. Chutes and Ladders is more technical, but once you're through the tough stuff, you will fly through the singletrack (I hit 23 MPH on this section -- pretty good on singletrack for an "old fogey"). Joes Ridge and Zipity Do Dah have steep tricky descents for a beginner but you can ride those while your wife is having a blast on Kessels.
There, a whole summers worth of bike riding. And there's so much I haven't even mentioned.
