Summer weather in the Rockies can be unpredictable. The warm, clear days and cool nights are wonderful. But afternoon rain and hail storms can develop quickly and unexpectedly, even on the nicest days. Carry rain gear and a sweater, as well as water and sunscreen.

Mountain air is dry and contains less oxygen. You may suffer temporarily from headaches, nausea or insomnia. Just take it easy for a day or so, and if you are in good health, the symptoms should disappear. BICYCLES: Twelve-speed touring bicycles and the currently popular, more rugged mountain bikes can be rented for about $10 per day in most mountain resort towns. I rented my 12-speed at a shop called Rec Sports in Breckenridge.

Summit Bicycle Tours offers cycling and lodging packages in the Breckenridge-Vail area, starting at $34 per person overnight for two nights and three days. For information: P.O Box 647, Frisco, Colo. 80443, (800) 525-9801.

Those interested in longer or guided bicycle trips in the Rockies might contact Timberline Bicycle Tours at 3261 S. Oneida Way, Denver, Colo. 80224, (303) 759-3804. Timberline offers guided tours ranging from three to eight days, from New Mexico to the Canadian Rockies. Costs range from $195 to $1,500 per person, including lodging or campsite fees and meals. WHERE TO STAY: We booked our bed-and-breakfast reservations through the "Bed and Breakfast Vail Valley" network, P.O. Box 491, Vail, Colo. 81658, (303) 949-1212. Listings range from traditional inns to rooms in luxurious Vail homes. Rates s range from about $30 to $50 per night for a double in summer, about half the ski season rate. My bunk in a dorm room at the Fireside Inn in Breckenridge cost $12. INFORMATION: Colorado Tourism Board, 1625 Broadway, Suite 1700, Denver, Colo. 80202, (303) 592-5410.