Put together little boxes or bags of toys and activities that you can pull out in emergencies. So maybe contain a small toy (dinosaur or similar), some different stickers, some magnets or something to put together, some flashcards, a mini book, a random item they can imagine as something else, something for counting or sorting etc.
Toy rotation in general so you can always grab there attention with something pulled from the cupboard.
Allowing one cupboard in the kitchen that they can pull all saucepans etc. from and entertain themselves.
Messy play but as contained as possible. So a big tray on a mat with lentils, rice, pasta etc. And pouring things.
If no worries about something messier then water pouring in the sink or kitchen. I once involved tea bags too so ds could make 'tea'.
Bubble wrap and lots of it.
Ripping up paper and maybe sticking it afterwards.
Making 'treasure maps'. These are good for walks too, you can collect or spot things.
Reusable stickers that can go on windows.
Magnitiles
Marble run
A pot with a slot cut in the top and they can post bottle lids or pop poms etc.
Big cardboard boxes for colouring, playing in, making a house etc.
A pop up tent or homemade den.
Getting all the cushions off the sofa and doing gymnastics or just rolling around on.
Hmmmm that's all I can think of right now... my usual fall back is a movie on a rainy afternoon but ds is older than yours.