Absolutely no tennis shoes as we don't need our trails widened any more. If you're not willing to walk in the wet trail, don't bother going this time of year. There are good places in the South Platte area with Lost Creek Wilderness area probably the best. Try the Goose Creek Trailhead. Denver residents usually are training this time of year by hiking in the foothills of Jefferson and Boulder counties.
Rock climbing is in full swing except for when an occasional late spring snowstorm comes along.
Almost all of the climbing crags are down low with the higher crags being the realm of the serious alpine rock climbers. Good areas are anything around Boulder , the South Platte with Turkey Rocks providing good cracks, and Shelf Road or Rifle providing sport climbing. Lumpy Ridge in RMNP is too high for warm climbing at this time of year. Of course there is still skiing. That's probably Colorado's best kept secret.