Saturday May 20? I'll be there, too, that day Going alone is just fine. If you go to California Adventure, take advantage of the single rider's line. You practically walk right on, even if there is a lonG line.
Disneyland Park is as safe and comfortable as WDW, maybe more so, IMHO. It will be a little crowded, but you can still see the whole park in a day.
It's about 2 hours from downtown San Diego to Anaheim by bus, 90 to 105 minutes by car (75 minutes if you drive like a car thief). I prefer taking the Amtrak, but for a Saturday visit, the last train will force you to leave a little early. It departs about 10:50, so you'd need to leave the park by 10, at the latest, to get out front and catch a cab. Then, that last train doesn't pull into the old Santa Fe station in San Diego until almost 1 a.m.
If you have the means, I'd consider taking the train to Anaheim, catching a cab for the last 2 miles to the park, and crashing at a local motel (there are many right across the street) for the night after enjoying a full day at the park. You can stay as late as midnight. But that's just me.
If you do go by train, the first daily out of San Diego leaves at 6:15 a.m. and arrives at 8:07 in Anaheim. That ALMOST gets you there for the 8 a.m. opening. In reality, you'll probably be on Main Street by about 8:45. The train ride, especially after the first half hour or so, offers wonderful views of the Pacific Ocean, which will be full of surfers all the way to San Clemente.
Oh, and make sure you get the number of the cab company when they drop you off in the morning. Plan to call them in advance. Just flagging one down could take a very long time. Unlike Vegas or New York, Anaheim is not a huge cab town.
You could take the bus in the morning, too, if you are planning to arrive on that first train. There are express buses (Route #430) that will take you from Anaheim Station to Disneyland ($1.25 / a 12 minute ride). These buses depart Anaheim rail station at 6:31, 7:01, 7:237, 7:52, 8:31, and 8:45. Don't bother taking the bus back, though, unless you are leaving around dinner time.
Having said all that, the SD Zoo is one of the world's finest. You can't go wrong there, either, and it won't require you to get an extra hotel room or stay up until 1 am....
