Since you specifically mentioned Boulder (I live a town over) I can suggest the following:
The 28th street mall and across the street are a kind of outdoor haven. Right next to the Montbell flagship is the northface store which is sometimes worth a look. We also have the REI flagship in denver, or the REI just across the street from Montbell is a new one that is fairly well stocked too.
Depending on your timing, you might be able to catch a GoLite or Spyder warehouse sale.
A little south from there is Neptune Mountaineering which carries odd-ball stuff as well as being a Mountaineering museum pretty much with the stuff they have hanging around - they even have a bench made from ice axes and belay devices.
For UL backpacking though, Montbell has got the most unusual gear and they have a pretty well stocked store.
As far as day hikes, hard to even know where to start as there are a dozen you can start a mile or two from those stores and hundreds within 10 miles. A lot of it depends if you want a summit (with the associated steep climb) or longer mileage. Within an hour or so driving, you've got the Indian Peaks Wilderness area and Rocky Mountain National Park - both of which have hundreds of miles of suitable day hikes. Oh, Eldorado Canyon is very close too...
For an easy-ish summit one you can start in town, check out the Bear Mountain Trail. It's only like 6 miles, but beware though, the altitude is likely to kick in if you do this trail on your first day or two.
If you want something more remote with a lot less steepness, check out some of the trails around Brainard Lake north of Nederland (take Canyon west out of boulder next to the REI). The altitude is much higher here (9-12K as a guess), but it's relatively flat and has grand views as it's located on the Peak to Peak highway.
Keep in mind, most of the population in the state hikes, so depending on when you go, don't expect solitude on day hikes, especially near towns, it takes longer trips or more remote areas to leave the crowds behind.
Whatever you decide, call the rangers ahead to check trail conditions. April may be 80 degrees but there still may be 5 foot slush piles and frozen drifts on the trails and it can rapidly change from summer weather to snowing.