Mines and caves: click for a cool video about a goldmine in Juneau!
An abandoned gold mine on an Alaskan island.
Mine tunnels are fun in the winter!
Repeater towers are fun to climb on, but watch out for the microwave radiation!
Here's a map of Sitka showing all the urban exploration related areas I know of, and some that are probably just local myth.
Here's a better map in which you can tell which way is north (up) and it's flat so you can see where things are.