After spaying how soon to allow normal activity?

I have never tried to restrict the post-op activity of any cat. I figure that they're the ones who had surgery, so they should know what hurts and what doesn't. Now, if she hasn't been introduced to your other cats before this, I would be worried about fights and other skirmishes, so I probably wouldn't try introductions until the incision is fully healed (around 2 weeks). But if they're already friends I wouldn't worry about it. Do you have a spare room she could stay in? Might be nicer than staying in the cage for 2 weeks.

I don't think restricting activity to avoid jumping and stuff like that is necessary. With the stray and feral cats I've had fixed they are released on the street after two days. My own cats and my fosters are just kept secluded the day of the surgery since they are still groggy but after that they are back to normal and running around. So far I have never had any problems with female cats resuming normal activity right away and through the years I've probably had at least 30-40 female cats fixed. Keeping her in a cage for two weeks sounds like a bit much. I really don't think you need to worry about any damage to the incision area or anything else.

Could you keep her secluded in a room instead, like the bedroom? After four days she is okay to jump up on the bed. As long as you supervise the interaction with the other cats it should be okay. A bit of hissing is normal and will usually subside after a day or two once they get used to each others' presence. So far I haven't seen any actual fighting between a kitten and an adult although sometimes the adult will try to paw the kitten if it comes to close.
I find that feeding the cats some yummy food together in the same room, although not too close, helps them associate the others with good stuff. In the beginning you can feed them on opposite sides of a door to get them used to each others' smell. Next meal you open the door a crack so they can see each other. After that they can be fed in the same room. If they can be in the same room without a bunch of hissing and posturing you can have them all together in a room as long as you're in it, like at night in the living room when you're watching TV.

I have never had to restrict activity with my cats after spaying. I did have to watch her jumping too much but it really was not necessary. Seclusion is probably the best bet if you have a "safe" room. You can isolate her for a couple of days and then allow her back into the house. If that's not possible, keep a close eye on her for a couple of days and see how she's doing.

Try as I might I couldn't stop my cats/kittens from doing what they wanted. I just let them be and kept a close eye on the incisions and also made sure that they weren't licking and biting the sutures.

Glad things are going so well sophielew!

Glad yours are doing well.
My gal Lotti would not stop doing what kittens want to be doing with her brother, after about 5 days part of her wound opened, I nearly went mad trying to stop them playing and her licking it.
In the end after a staple, antibiotics in case, today is the first blood free day!
14 days today!
The last three days I have just let her do her thing as vet took out staple and said not to worry anymore.
So I would advise to still keep an eye on that wound until the 14 days as skin will not be knitted properly until then.