Some activities I'd suggest for summer conditioning which provide good cross training for skiing would be mountain biking, road biking and roller blading.
I understand there are some "rail to trails" conversions in Hancock along the Potomac. An hour skate a couple of times a week is great for both aerobic conditioning, leg and core strength and balance.
The best place locally to get blades is probably Dicks, next to Home Depot in Hangerstown. I'm a fan of roller blading as its pretty simple and you won't get distracted by all the various bits and pieces that need to get repaired, added, adjusted on bikes.
Any summer sports activities are also good, soccer in particular, most baseball programs do not qualify as conditioning activities for obvious reasons.
Mountain biking is good, single track in particular as it forces one to keep looking forward.
Another option would be regular trips to the gym, working on building an aerobic base and then working on general strength.
One thing to keep in mind is to establish a routine which can be continued through the fall and especially through thanksgiving and Christmass holidays. Overall strength declines in excess of 1% per day during periods of inactivity. Its very easy in our environment to be in great shape in the middle of the fall and then have lost all that was gained by the time we start racing in January.