Tacky to send with the invitation, but...
There is a toy store here that has a great birthday solution. Because many of the customers shop there for birthday presents, (it's small and local, not a big chain) the birthday child goes in a few weeks before the party invitations go out, and chooses several gifts in all price ranges. The gifts themselves are stored in a big plastic tub with the birthday child's name, and anyone looking for a gift for that child has some choices. Of course, no one is required to choose a gift from the tub, and there have been times when we've chosen something completely different.
This isn't sent out with the invitation - it's more a matter of, "Oh, so and so is having a birthday. We'll go see what toy store X has for him." It's a nice service. The toys go back on the shelves if they're not chosen (usually there are multiples of everything in stock, so no problem about pulling merchandise off the shelf.)