Deskwarming

Create English materials and games - or research some ideas to get your gears turning.

Make an English Board. Check out this page to get some ideas from a fellow block member.

Study Japanese...or English!

Ask your JTE or HRT if there's anything you can do - chances are, there is.

Ask other teachers if there's anything they may need help with.

Get involved in other events or classes happening at your school.

Clean or reorganize your space.

Clean the school.

Take a coffee or tea break - but don't do this one too frequently or for too long.

Talk with other teachers - only if it isn't interfering with their work (you can use this as a chance to practice your Japanese or to give them a chance to practice English).

Read the news (in English or Japanese).

Check out this website and other JET websites for information on things you can do to level up your ALTness.

Watch movies or anime at your desk.

Play games on your smartphone or computer.

Sleep (I know other teachers sleep at their desks sometimes...don't do it).

Listen to music - unless you're researching songs to be used for English class.

Eat at your desk - go to the break room for that. In the case that you don't have a break room, pay attention to what the other teachers in the office do in regards to this.

Blatantly scroll Facebook or other social media - if your office is a laid back work environment, checking it periodically may be okay.

Make blatant vacation plans - maybe if you're that anxious, you can make some lists or research the travel destination and use the information for an English activity. Looking things up and being interested is fine.

Book flights and hotels for your vacation.

Organize your purse or wallet.