Are you flexible with your dates at all? I would highly recommend NOT going in April or early May. There will still be ALOT of snow, much of it unstable, and many trailhead closures will still be in effect. There are some great snow climbs to be had then, but you need to be very very cautious with the snowpack and make sure to get alpine starts (ie. get off the mountain before it warms and starts falling on you).
If April and early May it is, I would suggest checking out Quandary as a good warm-up hike--not too hard, open trailhead, and close to Breckenridge. You can contemplate your altitude readiness while bar-hopping in Breck.
If you come a little later, and the road is open, there are two great routes on the aforementioned Torreys Peak that avoid the standard, popular (easy) route -- Kelso Ridge and Dead Dog Couloir. The Ridge is a class 3 (easy 4, maybe) scramble that isn't too exposed except for one short section. The Couloir is a 45-50-degree snow climb -- very fun but requires an early start (easy-facing). Torreys is close to Denver so it makes a good test piece early in your stay.
I personally don't think the Collegiate Peaks (the peaks near Leadville and BV)are all that interesting in terms of challenge, but they do tend to be more accessible for early season mountaineering and the views are sensational. Bring skis while you're at it b/c there are some great possibilities! You should look into trying some of the Sangre de Cristos, too, esp. in May. Many routes melt out fairy early and they are amazing mountains. Crestone Peak and Needle are especially cool; the better approach is from the west, unless you want to try Humboldt, too.
The most interesting peaks are also the ones that are probably out of range (due to wet, slab avalanches) in the early spring -- the Elks outside Aspen and the San Juans in SW Colorado. Also, Longs Peak, definitely a classic, is probably off-limits that early. I've turned back on Longs in June due to too much snow.
PM me with any questions; I just moved back to Vermont after living in Colorado for a while.
In addition to the other suggestions, check out these guys' website:
http://www.ryananderin.org/index.html
and some of my own pics from this past May... (I hope the links work!)
Early May 2006 on Quandary
here
Memorial weekend on Democrat, Cameron, Lincoln, and Bross (the Decalibron)
here.
Memorial weekend on Evans, Bierstadt, and Spaulding
here.