It depends on how much trouble you're in. For normal miss understandings, some form of chocolate is always a good idea. Women love it and dark chocolate stimulates our brains to experience happiness. If you've really messed up, you might need to buy her a new car.
I love the spa idea. Give her a day out. Take the kids and leave. Take day trips just the two of you and most important find out why she is mad and tell her you love her and will try to change. i text my wife "Your damn sexy"
Trying to "buy" her back is probably not going to work (unless she is superficial). I think you are much better off getting her something that shows how important she is to you. Something you make yourself accompanied with a very thoughtful card should be helpful.
Get her something she has talked about wanting to get for herself, whether it be a piece of jewelry, a book, electronic item, etc. If there is nothing she has asked for make her a nice meal or take her out to a nice restaurant.
A box of low sugar chocolates. Making a repair to something she has asked to have fixed, without her having to request it again. Buying something from her personal wishlist.