The frying Pan is a great choice any time of year and its only about a half hour from Aspen. The last time I was there I had four days of excellant fishing. If you are not looking to catch a lot of fish but would like to catch some really huge ones, Mysis Shrimp are productive in the section of river directly below Ruedi Resevoir. Expect to share the water with a lot of other fisherman on this stretch of river. You can also fish any of the numerous hole further down the river as well. You will probably catch a lot more fish in these areas but they will not be nearly as big, although a 15-17 inch brown is nothing to sneer at. Don't bother with the mysis shrimp if your not fishing right below Ruedi. A good set up down river is a #14 or #16 prince nymph about 3 feet below a split shot with a #18 or so copper john trailing behind the prince about 2 feet. There is usually not much in the way of dry flies this time of year but there are some small hatches. The fly shop in Basalt is a great source of info.
Lucky you indeed! I lived in Glenwood Springs for two years prior two moving to WA. I fished the Frying Pan all winter long, and it is all it has been said to be in the other posts here. Use mysis for the hogs up to 12 lbs or so in the first 500 yrds below the dam. Expect company in this streach, you may be shoulder to shoulder with the trophy hunter subculture in the toilet bowl and bend pool. These fish are caught and released a lot and start to act like trained seals. Plenty of good fish downstream, nice water and still some astonishingly big rainbows that seem to migrate downstream and back. This time of year you will do well with midge larva, black and red. WD40 and black beauties will catch lots of fish (small, 18-24). Pheasant tails are also clutch, I used to tie them black and these outfished brown for me. Again go small, 18-20! This water is often gin clear so 6x fluorocarbon or 7x mono, and keep casting, some days they wont move out of their feeding lanes. The canyon is spectacular, but much of it above 8,000' so bring your neoprene, dress warm and expect snow.
The Roaring fork is also world class, but a freestone, so depending on water levels could also be great. It flows from Aspen to Glenwood. Prince nymphs, midges, and despite what you are told, stoneflies will tie you into some nice bows and browns that don't take kindly to being hooked. Best of luck to you!