Home visiting teacher had a pig that came with a load of gold 'coins' that you put into a slot on its back. About every 4-6 turns, it made a loud oik. She used to sit one or other of them down and do turn taking with them. I loved it so much I bought one and did it with them too!
I also bought this game where you put a ball at the top of a helter skelter and it whizzed down and into one of 3 slots, where it knocked a penguin down. We took turns on that too.
We had animal sorting, colour sorting, shape sorting. I blew bubbles but they had to say "go" to get me to blow!
I taught them how to do facial expressions. Sat on the floor and said "happy face" and then grinned like a loon. "Sad face" and then did that, "cross face" etc etc. Over and over and over. They enjoyed that (sometimes) and it also taught them something!
card matching is another one. 10 cards (5 pairs of whatever - 2 ducks, 2 dogs, 2 rabbits....) face down and random. Get them to turn one over and then another, if they don't match, turn them back. Then it's your turn..keep going until you've paired them all up. This is something we only do now, they couldn't understand the concept until now. ds2 still tries to turn them all over but ds1 understands.
Dominoes - I've got some with pictures on. They like matching things up.
Skittles is fun too.
Quite often, especially when they were younger, I ended up playing on my own! But I carry on. Maybe they'll come back, maybe they won't, maybe they'll be watching and taking it all in. Whatever, I'm happily colouring in, painting or making playdo people
I find you can also unite in rude noises. autistic or nt, nothing pleases a small boy like farting sounds.
My 2 are very physical, so throwing them about (when they are in the mood) is always a winner.
And finally, backing off is fine too. Sometimes they just don't want you. They are happy doing their own thing. It's important to not be in their face too much.