In response to Andrei's request for just one good problem I have used, I shall share one activity that my students enjoyed. (I did read a hint of sarcasm in the request, but I'm sure I was reading beteen the lines.)
David Foster, a dear friend of mine and author wrote a unit titled Home Improvement. This is a part of the unit. The students are given costs of several items needed for redecorating. The cost of a gallon of paint with how many sqaure feet can be painted with a gallon, the cost for wall paper, various floor coverings etc. are given and entered into a spreadsheet. Many of the items are available in multiple grades. The students are given the task to remodel people's homes according to the needs of their clients. To do so, they must enter formulas to find the cost of each item for a square foot. A spreadsheet is designed to track the costs as they chage parameters. They worked through several different designs, using the spreadsheet to model changes. Their working knowledge of formulas increased dramatically. AND they understood what they were doing. This really is a wonderful unit. I used it with my eighth grade algebra class. If you are interested in more information,feel free to contact me. Send me private note at firstname.lastname@example.org so this list won't get cluttered.