Replacement exercise for step-ups

I have this workout scheduled for later:

It has both regular step-ups and side step-ups. Problem is, I don't have a step! I used to use the stairs in my old place but now I live in a ground floor flat and I don't have anything sturdy enough that I trust!

What would be the best exercise to replace them with? I was thinking lunges but I know there's people on here that are more knowledgeable about which muscles are worked etc.

Thank you!