I did my first Disney Trip in January as a solo. It was a lot of fun!
My biggest piece of advice is to know what you like and be sure to plan any of your must-do things well in advance. So if there's a specific restaurant, tour or show (like Hoop-Dee-Doo) that you want, be sure to book it as soon as you can. Certain things, like Cinderella's Royal Table can get hard to get into and certain rides like Flight of Passage get all of their fast passes taken the first day.
I'm a planner, so I planned every day down pretty good and it worked really well for me. I know that this strategy may not work for everyone. Also, be sure to get your Fast Passes picked as soon as possible. You can change your mind later. Also, the earlier you get your 3 fastpasses done in a day, the sooner you can use your extra fastpasses. You can keep getting more until the park closes and you can register for a new one as soon as you scan your magic band at the ride.
If you have the endurance, get there before rope drop and drag your feet after the park closes. The busses will be packed if you go straight there. Next time, I'm bringing disposable face masks for the trips.
-Animal Kingdom
Your first priority should be a fast pass to Flight of Passage. Kilimanjaro Safari is best at rope drop since that's when the animals are most active, the line is non-existent and the cars go the slowest. Be sure to utilize the singles line for the Expedition Everest rollercoaster.
Tiffins is my favorite restaurant at Animal Kingdom, by far. Pongu Pongu in Pandora has my favorite snack, the non-alcoholic Night Blossom.
-Epcot
I wouldn't use a fast-pass for Mission Space. I don't care for it and the line was a walk-on by the time the park was near closing time. It was also January. The line to the Frozen ride gets really short near closing, so it may be a good time to ride it then. Soarin' also has no line near the end of the day.
My favorite restaurants at Epcot were Chefs de France, Askerhaus Princess Breakfast and Garden Grill.
-Magic Kingdom
The rollercoasters are all great to use your fast passes, but Big Mountain Thunder Railroad's line gets pretty short near the end of the day. Seven Dwarfs Mine Train and Space Mountain were always long, but they were shorter at road drop.
I loved Cinderella's Royal Table and the Dole Whip is really good.
-Hollywood Studios
This is kind of a weird park for me. They were under a lot of construction while I was there, but Toy Story Land will be open by the time you get there. Next time I go I'm probably going to try to use one fast pass for the Slinkey-Dog Coaster, one on Toy Story Mania (if possible) and Rock n Roller Coaster. Be sure to use the singles line in Rock n Roller Coaster.
I liked the Prime Time Café the best, even more so than the much more expensive Brown Hat Derby.
Most of all, have fun and know thyself. If you want to be more relaxed than me, be more relaxed. If you want to get up at rope drop and end the day after Midnight like I did with the Disney After Dark thing in January, do that. It's your vacation.