Abandoned Cinnabar Idaho Mine Site (watch)
The abandoned Cinnabar Idaho mercury mine would make for a worthwhile summer adventure.
This one doesn't look easy to get to and those that have already been there, recommend ATV's or motorcycles as the best mode of transportation to get there.
The mine is located about 15 miles east of the unincorporated town of Yellow Pine, Idaho with a population of a whopping 32. Here are some jumping off points that you can search on Google Maps to get a good idea of how to get there. The exact coordinates and some pretty detailed directions to Cinnabar can be found here.
A word of warning. This info from the EPA states that there are some contaminants of concern, including mercury, methyl mercury and arsenic.