How have you dealt with baby gates and blocking things off from the baby, when you want or don't mind your toddler having access?

In particular right now I'm thinking about our stairs. Our 3 year old is fine on them and likes to go up to her room or whatever, and i am ok with that. In the morning or after nap, she comes downstairs if I'm down there already. This is nice for me to not have to get up and get her.

But the 9 month old is getting more mobile and is NOT ok on the stairs. So I'm curious how people have dealt with that. So far the baby has a "baby jail" where she is completely safe when I have to walk away, but I know there will be a decent chunk of time when she's too big for her jail but not old enough to do stairs yet.