Upcycle your old jeans - check out https://www.levistrauss.com for ideas.
Gardening - grow some veggies, flowering plants or succulents. Use pots if you don't have a garden.
Make up your own dance routine - be creative, it doesn't matter if you've never had a dance lesson.
Swim - visit the local pool and swim. If you can't swim well, walking in water is good exercise too.
Camp in the backyard - pitch a tent or sleep under the stars (make sure you use insect repellant).
Play a board game - try a new game. Borrow one from a friend or relative.
Write a novel - if writing a novel is too daunting, try a poem or short story.
Learn to cook - taste.com.au has a Youtube channel
Learn a new language - try a free app or website like Duolingo or Memrise.
Create - buy some air dry modelling clay, do some paper craft, make some jewellery, get creative.
Journal - grab a notebook and pen and write about your feelings, your goals, your fears, your hopes...
Go to the beach - swim, make a sandcastle, look for shells, play beach cricket or volleyball...
Draw - even if you think you can't, give it a go. There are lots of tutorials on YouTube - here's one on how to draw a face https://youtu.be/Fhfx35Lwux8
Go for a bike ride - self explanatory really
Go roller-skating - there indoor roller-skating facilities in most cities
Learn to play a musical instrument - you can hire musical instruments if you can't afford to buy one.
Paint - you can buy cheap paint sets from K-Mart and find hundreds of tutorials on YouTube.
Go Geocaching - it's like a treasure hunt. Here's an explanatory video https://youtu.be/vuFiLhhCNww
Make a bucket list - make a list of 100 things you'd like to do before you're 100.
Declutter your room - choose a small area of your room and make three piles: keep, donate, throw out. Repeat.
Learn to sew - if you don't have a sewing machine at home perhaps you can borrow one. Or you can sew something by hand.
Do a scavenger hunt with your friends - grab some friends do an alphabet scavenger hunt (take a photo of something beginning with each letter of the alphabet)
Paint your nails - no explanation needed. It's more fun when you do it with family or friends
Make healthy smoothies - check out these 12 healthy smoothie recipes at https://youtu.be/YuDhbLQtt2k
Design your own Lego model - find lots of inspiration at Lego Masters Australia
DIY day spa - Anna McNulty has a day spa video with DIY recipes https://youtu.be/bCyLczCh2uc
Visit your local library - borrow a novel, a non-fiction book, a magazine, do a community class...
Learn yoga - Kassandra has a 10 minute video for beginners https://youtu.be/yigadoKHUVM
Note: some of these won't be possible if you're in quarantine or lockdown - please obey your local rules and regulations.