Well, if you have to be somewhere else on May 8 and you need 2 weeks before to prepare for the race, you're talking about being in Boulder, CO around mid-April which basically is still winter in the mountains.
For instance, Rocky Mountain Nat'l Park which is near your son is open year-round, mainly for hiking or cross-country skiing now. The campgrounds open May 21, weather permitting. The main road all the way through - Trail Ridge Rd. opens May 22, weather permitting. So you probably won't be able to see that park although you could get to Estes Park, just outside the park.
For mid-April you could maybe first head for Arizona and take in Grand Canyon and then Utah's Zion Nat'l Park and maybe even Arches Nat'l Park but most others would still be iffy, weatherwise. Then visit your son in Boulder, CO and then head home.
You've got a long drive and three weeks is going to go fast. If you're retired, why not do the trip when you don't have other commitments and can take more time?
