They love water play at that age. Stick a swim nappy on and sit them in an empty bath with a bowl of warm water and some toys. One day it can be plain water, the next time add some food colouring, the next time have bubbles..... you can also give them a couple of cups with different coloured water in and a big bowl so they can do mixing. Bath pipes are great - they have suckers so you can stick them on the side of the bath for your kid to pour stuff through. If you want more ideas, you can freeze some little toys in ice cubes and give your kid warm water to pour over them to “rescue”them. Basically there’s a tonne of water play/messy play ideas out there’s and by doing them in the bath in just a nappy you have virtually no clear up time! Obviously be careful of bath getting slippy - we had one of those plastic bath mat thingies.
Seconding balloons. We got a cheap pop up ball pond and chang3 what was in it each day - balls all one colour one day, mixture of different colours next day, all different sizes and textures another day, balloons... just keep mixing it up.
Mine absolutely loved mixing things. Mashing up weetabix was. An absolute winner. Made it into something afterwards if I could be bothered!
Sensory bottles can be fun, but might be more effort than they’re worth as you can’t tell in advance what your kid will like. BasicLly old plastic bottles. Tricolour pasta in one, rice in another (you can dye it with food dye if you like), coloured sequins, coloured beads.. each one makes a different noise. Make sure you glue the lid on so there’s no choking hazard. My DD used to shove them and roll them across the floor or I’d try to get her to shake them to music. You can do liquid ones with glycerol etc to make layers, but although they looked pretty they took effort to make and DD didn’t like them as much.
Do some dancing if your DC isn’t too heavy to easily hold. If they can bear most of their weight on their legs hold them firmly and encourage them to boogie to music. Or just dance around them when they sit on the floor. Get a disco light or projector if you really want to go wild 
Some of these ideas require small amount of prep so won’t help you today, but there’s plenty of damp days coming up!