The weather in this area is beautiful in the summer. We expect highs in the mid 80's and lows in the 50's around these dates. The humidity will be almost non-existent, and there is usually a nice breeze coming off of the continental divide. Be prepared for the possibility of brief rain showers in the afternoons.
If you're wanting to take some time to explore the area and find some excitement, that's great! For hiking you'll obviously want some sturdy shoes or hiking boots and a comfortable pair of shorts or pants, probably not jeans. If you're wanting to raft, bring a swimsuit and some shorts, maybe a t-shirt (no cotton!); the water is chilly but you get used to it pretty quickly. Regardless of what you're doing, sunscreen is a must. Buena Vista is almost always sunny in the summer and the high altitude (8,000 ft.) and thin air mean that you get burned more quickly than you would at a lower altitude. You'll also want to make sure you're drinking lots of water, as it's easy to get dehydrated at altitude, which is a quick way to ruin your day.