Gestation is 63 days (or 8 weeks) for cats, so it's not a long period. Just as we experience, the activity level does go down around week 6/7 because it's difficult to move around, so that's normal. Week 8 she'll start nesting and increase her activity a little bit. When she starts stage 1 labor, she'll probably become extremely loving...to the point it is annoying. It's a rare cat that wants to be left alone in stage 1. Just pet her and talk to her when you see her, and keep an eye on her.
If she's familiar with the bedding box, she'll probably queen in there (give birth). Sometimes they go for a private area, but she already knows this is a safe and fairly warm location, so chances are she'll choose it. When she does queen, keep an eye, make sure she doesn't need help. Generally, this is a rare thing and people helping only impede nature, but sometimes a dystocia happens (kitten gets stuck). DON'T FRET ABOUT THIS because it really isn't a common occurance in domestic cats, just take a look at her once in a while to make sure she's okay. If she's crying while pushing, don't panic. It hurts them like it hurts us, and sometimes screaming just helps with pushing. She's just saying "come on, kid! Get out here! Ouch!"
Once she's queened and the kittens are here (expect at least 4 in the litter, but don't be surprised to see 8 or more), make sure your kids don't bother the kittens. Momma can move them when she feels they're in danger, and she won't hesitate to. They can see, but no handling for about 10 days (you can handle a little bit, but just a little bit). They'll have their eyes closed for a couple of weeks and be more rolling in movement than walking in movement. You *might* want to place a heating pad on low under the bedding or box, or get a heat lamp to help keep the kittens warm since the nights are cold and in case cold weather moves in.
If you want to get an idea of where mama kitty stands in terms of giving birth, see if she'll let you check her milk. When it is bright white and rich and no longer with a yellowish color or sticky, she's very, very close.
After the kittens wean (about 6-8 weeks) and her milk dries up (about 10 weeks) take her to the low-cost clinic and get her spayed and shots. It's not a lot of money, will prevent more pregnancies, keep her healthier, and help prevent the spread of feline disease. Through the low-cost clinic it will be less than $100.
Good luck, have fun, and good for you to take in a stray! :)