I'm listing a few things off the top of my head that might be fun for the 8-year-old to help with as well. These are the low cost varieties! I think out of all the toys that have been bought for my children, they seemed to liked/use the following the most (probably in order of best being at the top):

• The toilet (it's fascinating and kids learn what floats and what doesn't--I don't particularly like this one though!)
• Together time (especially outside but inside can be a lot of fun too if you take some time for that one-on-one)
• Empty boxes (You can check with a local furniture store to see if they would give you a refrigerator box if you want to go with the big guns and turn one into a playhouse. The diaper boxes we have can double as a train or you can cover with cloth and use as a toybox. My kids usually get more playing time out of the box then they do with an expensive toy I thought they would just love)
• Turkey baster, big spoons, pots--anything in the cabinets seems to be what my youngest would love to have.
• Books from the library (free is always good and you can also find music CDs for car rides or for playing around the house) 
• Bubbles (I recommend getting fairly small sized bottles because they often lose their ability to make good bubbles if they sit around the house for long and they are easier to travel with)
• Balls (any safe size or type with or without a game)
• Toy Library (Our county's Smart Start offers themed kits where any parent of a preschool aged child can check out kits filled with books, puzzles, games, or other hands-on activities. You may wish to see if your county does the same thing. Then you'll be able to find out what really interests your child before you go out and buy something that he/she will play with once and then toss to the side--been there and done that many times. Also, if the toy is really annoying to you but the child loves it, you only have to listen to it for a few weeks!)

Hope that may help a little bit. I've found that simple is often the best way to go. I try to purge the toybox every once in a while and either rotate them around to the basement (look, Mom--a new toy! is actually just an old one that's been stored for a while) or donate them. Each child is unique and tends to gravitate toward different things. For example, if the child loves to climb on everything, then active toys that allow for some type of climbing are excellent.