No biology?
Buy some cut flowers. Stand them in water containing some ink of a colour that contrasts with the flowers. How long will it take for the ink to appear in the petals? Why do we stand flowers in water? What happens if you take them out of the water? Why does that happen?
Get a bird feeder and hang it outside your window. What birds can you see? What foods do they prefer to eat? Why do they seem keener to eat at some times of day than others? Find out whether they are residents or migrants. If they're migrants, where do they migrate to or from? How did they find out where they migrate to or from?
Buy some "trip trap" mouse traps and set them in your garden (follow the instructions they come with). Check them morning and evening. When you've caught something, tip it out into a big transparent plastic bag and take a look (be careful, lots of small mammals bite, hard). Isn't he cute? Is he a he, or a she? How can we tell?
Buy a microscope. You can get quite nice ones that plug into a computer, or the hand-held kind where a slide is held on with a ring magnet. Have a look at pond water, blood, skin, hairs...
Look a tree stump or a cut log. Why are there rings in it? How did they get there? How old was the tree? Why are some rings thicker than others?
Are there worms in your garden? Or compost heap? Make a solution of mustard and water a small patch. Now how many worms do you think there are? Why did they suddenly appear? What eats worms? What do worms eat?
Minibeasts. You won't have to look far to find an earwig, or woodlice, or a beetle or two. What do they eat? What eats them? Have a good look at them with a magnifying glass.
Grow something. A broad bean is good. If you just put it in a plastic cup on a bit of damp cotton wool, you can see the roots and shoots growing. Have a look and measure it every day. How fast does it grow? Does it grow better if you keep it in the dark? Does it grow faster at night or during the day (beans do grow quite quick)?
Go out on a clear night with some binoculars and do some astronomy. Look at the moon. Check out Mars and Venus. Can you name some of the constellations? If you live somewhere really dark, you'll be able to see the Milky Way. What is it? (If you live in a city, save this one for when you're on holiday somewhere where it does get really dark. You can see the moon and the brighter stars pretty much anywhere though).
What direction is a really bright star in? Find out with a compass. Is it still in the same place an hour or so later? And another hour? Why has it moved?
What about the sun? Does that move?
Make a crude sundial with a stick stuck in the ground. Put another stick or a chalk mark or whatever where the shadow falls. Has the shadow moved an hour later? Mark it now. And again, and again. Could we tell the time with the sundial? Will it work again tomorrow (if it's sunny)?
You can have endless fun with a torch battery, a bulb and some bits of wire.