Colorado in June is awesome. There is a festival just about every weekend (Teva mountain games, FIBARK, local river festivals etc). Just about everything should be running, and every major drainage should have a good Class IV or V run. I second getting CRCII as it is essential for paddling in colorado and it seems tailored more for the IV-V crowd. The arkansas is a good first stop. FIBARK is a big party, the ark has Numbers and Pine Creek for IV-V action, and if the water is big Royal Gorge should be good too. Several good playholes in Salida and Buena Vista for park n play or an afternoon session, and lots of down river play on many sections of the river. Always lots of boaters to paddle with. Cache la Poudre has several good runs on it. Southwest colorado is awesome too. If you are by yourself and want to hook up with other boaters, go to the main spots like the Ark, Cache la Poudre, Durango etc. May is hit or miss as some of the rivers are going early, it might peak late may, but you can still have snow etc. June is primo. If you are an AW member, a cool article on colorado was published years ago about colorado summer trips that you can find by searching the online journal.