Get some friends together for a good sing, serious or silly, it doesn't matter. Music stimulates the brain, singing is good physically and emotionally, especially with other people.
Learn something new. There is a huge amount of online courses available for free, where you can efficiently take classes from top universities in the world, working in your own pace. Here's a few you can try.
You can also learn new language. There is a great (and absolutely free!) platform called Duolingo. Check it out!
1. Get some good binoculars - or better still a telescope - and look at the moon. 2. Go on the internet and find some great origami designs to make. 3. Write a short story or article and publish it online.