Colorado brags of 300 sunny days per year. In addition, the high altitude and low humidity means relatively little protection from ultraviolet rays. Bring sunglasses that protect against UV, and don't forget the sunscreen.
Toiletries and Medical Supplies: Sunscreen, mosquito repellent
Photo Equipment: The mountains are stunning, but fancy equipment is tough to carry while climbing the flatirons or hiking mountain parks. A small digital camera might be best.
Camping/Beach/Outdoor Gear: A sweater or light jacket is useful even in summer, given the change in temperature from morning to evening. If you want to blend in with the locals, opt for a fleece vest.
Miscellaneous: Most anything you've forgotten can be picked up at McGuckin Hardware, a favorite because it's locally-owned and community oriented. McGuckin's is not a bad place for people-watching, at that.
Bring good hiking boots. Sneakers are not good enough. It is very painful limping down from a hike with a sprained ankle. It is humiliating to be rescued by 25 overly enthusiastic boulder rescue volunteers.
It goes from hot to cold here very quickly. Wear Layers: You want capilene, light or heavy weight or a similar fabric that wicks away moisture while keeping you warm. It is also helpful to have a layer to repel snow or rain to go on top. Those nifty raincoats that cram into one pocket of the raincoat are handy.
Toiletries and Medical Supplies: Sunblock is key here. We get amazing sunburns due to our altitude. Bug repellant is very important in late Spring and Summer because we have had West Nile problems with mosquitos. Use alot of both!
A good first aid kit is wise if you plan to hike much; it should have an ace bandage, antiseptic wipes and bandaids, ibuprofen and tylenol.. Typical injuries are road rash from crashing on mountain bikes, twisted ankles and bruises and scrapes from falls while climbing.
Camping/Beach/Outdoor Gear: You should have a below zero bag if you plan to camp. Even in July you can get a blizzard in the high country. Make sure you have a three season tent for most conditions just in case. A stove is good, of course. Bring something to treat water unless you plan to boil all your drinking water. Bring your compass and get good maps.
Miscellaneous: If you plan to backcountry ski, for heaven sake don't let anybody rent you a goofy little touring package with the thin skis, flimsy pools and little elf boots. You will hate back country skiing for the rest of your life. Instead rent fatter skiis with metal edges so you can carve turns, get leather boots that don't have too much flex so you have ankle protection and support and can make turns, get telescoping poles if possible with self arrest grips if you plan to ski anything steep.
Hiking boots, Fleece, clothes you can layer. People here are obsessed with these rubber shoes called "Crocks."
Toiletries and Medical Supplies: lip balm! carry it everywhere and have it at all times. Also - sunblock!!!
Camping/Beach/Outdoor Gear: A "Camelback" or water bottle for hiking
Toiletries and Medical Supplies:
Bring sunscreen...you're up nice and close to the sun in the mountains. Protect yourself!
Bring moisturizer...for everything! Your lips will chap and your skin will turn to dust without some sort of moisture being applied.
Miscellaneous: Water. Carry it wherever you go.
Clothing/Shoes/Weather Gear: Be prepared for anything, weather-wise. The saying in Boulder is, 'if you don't like the weather, just wait 15 minutes!' You can wear shorts in January, or it can snow in May -- you really never know what to expect. But, no matter what the weather, you can count on the sun coming out again soon.