For about 300 bucks you can replace you cabin speakers and HU. It took me a couple minutes to search the web.
I guess if it were me I would make sure all the maintinence is caught up. All that little stuff that people never seem to get around to doing. A good tranny flush is a must. Your Sable will thank you. Maybe take it to a detailing place and get it all cleaned up. Shampoo the carpets and seats, make it look like new again. Like the others have said, tinting the windows is always good. Gen 2's look great with tinted windows.
i would say the tint, and start saving for rims. rims make a car.