Sick days are for lazy day fun.
1. Start a New TV Show
When you're sick everyone just wants to watch tv. Preferably something you can binge watch. I would highly suggest Brooklyn 99 for your next sick day.
2. Drink a lot of water.
When you're sick, a big thing that happens is that your body often gets dehydrated. Drink as much water as you possibly can to keep yourself hydrated and feeling okay.
3. Sleep.
Your body is clearly tired and needs to recuperate and to relax. Use the time to fall asleep and take a couple of naps while you can.
4. Online Shopping.
Without even leaving the comfort of your bed, you can still do what many people like to do nowadays. Shop online. Some might splurge on themselves since it National Girlfriend's Day has come and gone whereas others might be working on decorating their apartment.
5. Keep everything clean.
As crazy as it sounds, cleaning is so important when you're sick. It allows you to keep your place sanitary, keep you feeling as well as you can, and it allows you to feel better after the fact. Cleaning allows you to keep the risk of infection for others low and allows you to not have as many risks when you feel better.
Sick days are often considered to be the worst. Taking advantage of them in what ways you can will benefit you.