I have taken many solo trips to WDW and enjoy doing different things each time. It really gives you a chance to do and see things at your own pace, either slow or fast.
Here's some of the stuff I've done solo:
* Thrill ride day - Hop from park to park and take in all the fast, big rides, using the Singles Line and Fast Pass to your advantage.
* Back Stage Tours - These are terrific. I once even had an Epcot tour to myself when nobody else showed up for the scheduled tour. A private tour for a regular price. Can't beat that.
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MVMCP and
MNSSHP - You can wizz through these special events and still enjoy it.
* Gourmet Tour - If you aren't there during the Food and Wine Festival you can still plan your own gourmet tour (or maybe a junk food tour). Epcot is the perfect place for this.
* Resort Day - I often do this on check-out day. If I have time to kill before going to the airport I soak in my last chance to sit by the pool and catch some rays. It's a relaxing ending to the trip.
* Safari Day - AK is a great park if you take your time with it. I select one day to just stroll around and stop along the way, taking pictures.
* Sports Day - If anything is going on at WWoS this is a great thing to take in. They have baseball spring training games and other competitions held there. There's also Richard Petty's Driving school for the thrill seekers. Of course, golf, fishing and tennis (and much more). You can hike through the grounds of the Wilderness Lodge and Campground. Rent a bike or water craft.
* PI Club Hopping - I usually enjoy the Jazz Club, Adventurers Club and Comedy Warehouse most of all and can do these at my leisure. Sometimes I'll add a bit of DTD shopping too.
* A day off WDW property - I'll drive to Celebration, Cocoa Beach, another theme park, whatever. It's nice to see the world outside the "world".