I know Miss Manners disagrees, but I tell people (especially family) exactly what they can get my kids. I honestly had to train my in-laws. They went overboard and not only did my kids not appreciate the gifts they chose, we had no room for the mass amount of stuff they wanted to buy. But then again, I've never been one to worry about tact. I'm the one that has to worry about what to do with the stuff and the fallout of an overstimulated kid, not the gift giver. A gift should never be a burden. Most people want to give gifts that are appreciated and will overlook a little breach in tact from an overwhelmed parent (let them think you're overwhelmed). Those that still insist on buying inappropriate things are doing it for their own selfish reasons, so I don't worry about insulting them too much.
For example, my youngest wasn't into the normal superhero movie tie-in toys, which is what everyone wanted to get him. He liked things he could build with from a very young age and he had advanced fine motor skills. I point blank told everyone to get him Legos, Thames engineering kits, K'nex, and the like, and to just ignore the recommended ages on the side of the package. Sets that you can add on to are the best gifts IMHO.
Currently, we send out requests for money toward big events. One son is saving up for a trip to Space camp, which will probably run near $1,500 with airfare. I let the kids tell the relatives when possible, that way their excitement makes the relatives want to buy the right thing or donate the money. His request was met with a huge box filled with astronaut ice cream, a NASA t-shirt, some space-themed books, and a check for $200. Sure, a lot of little things but they were all things that were consumable, useful, or wanted. I can deal with that. A Zoo or museum membership is an awesome gift to suggest, especially if the person giving it can take the kid there a few times.
Amazon wishlists are awesome for people that are internet savvy. Kid-written or dictated wishlists (with kid drawings on the paper for children who aren't yet writing) work great for non-internet relatives.
Be apologetic when people insist on getting stuff your kid doesn't like. "Oh, that was so generous of you to get little M that Barbie dreamhouse. Unfortunately she just wasn't into it but she loves the TMNT pizza play set she exchanged it for!" They'll get the hint eventually.