Between the snow-thick mountains, the buffet of winter activities (skiing to ski-biking to ice skating), the availability of heated pools, and the towns' small scales, both Vail and Aspen make for action-packed yet relaxing vacations. There's lots of vigorous exercise, but little schlepping. Your family will either be skiing in the mountains, playing in the snow, or hanging out by a pool.
Vail and Aspen change dramatically between winter and summer. In the winter, the towns revolve around the snow and the mountains. In the summer, mountain biking, river rafting, fishing, and hiking become the focus. Fall is a good time to visit, in part because it's much less crowded than the busy summer and winter seasons, and you can often get good deals on rooms. Plus, the shimmering gold Aspen leaves are a sight. Spring tends to be muddy and chilly, with the threat of snowstorms always present.
Because of the high elevations of both towns, the weather is awfully precocious. One winter day can be 55 degrees and brilliantly sunny, and the next, three degrees and blizzard-like. Most of the snow tends to fall after the New Year. Late winter into spring is the most tempestuous time, weather-wise, for Vail and Aspen. Summer can climb into the high '80s, but most days, the high temperatures sit in the '70s and low '80s. Summer thunderstorms are routine and can sweep in with little notice. Get more weather info.
Most people fly to Denver (DEN), rent a car, and drive the two hours along Interstate 70 straight to Vail. Aspen is another two hours past Vail (an hour further out on Interstate 70, then an hour south on Route 82). Amtrak trains are also available in Denver while Greyhound buses will take you to Vail.
Both Vail and Aspen are small towns; they can easily be walked from end to end, and neither requires mastering a mass-transit system. They both have shuttle services to help skiiers get around. Having a rental car is smart, especially if you plan to make excursions outside of the town limits.
Vail and Aspen offer festivals of all sorts. Here's a smattering: