What is the best way to spend alone time?
Start of your day with a glass of water.
Go for a walk/jog.
Come back home and rest for some time.
Then study something, trust me you will have very high concentration level. You can study anything from a novel to quantum physics.
Then have your breakfast.
(I suppose since you are asking for a whole day so no work/school/college)
Do something creative, write or draw or if reading is a hobby read.
Then watch a nice movie/TV serial.
Have your lunch.
Take a nap for an hour or two.
Wake up and go for a walk. Eat something you've wanted to eat from a long time.
Then enjoy the evening by calling an old friend.
Send messages to everyone who were once close to you but know you seldom talk with them.
Have your dinner and listen to your favorite song before going to sleep.
Hope this answers your question.