How to get Grandparents more involved w/ our kids?

I'm looking for some ideas to get a set of our parents (my kids' grandparents) more involved with our two (2 & 5) kids. The grandparents live out of state and do not realize how much we are disappointed that they don't show much of an interest in what's going on with our kids. I think the kids are mostly oblivious and think they are too young to know the difference, which isn't the case - our kids notice, too. I think if we were to mention something, the light bulb would go off for grandpa and grandma, but how do you do that tactfully? They don't put an effort into being present in the kids' lives (even through a postcard in the mail - or a birthday card) and then don't understand why the kids don't "know" them when they come to visit us every year and a half...I thought about asking grandpa if he would like to be penpals with our 5 year old for the summer - or something like that to get the ball rolling, but not sure. I'd LOVE to hear any ideas! Thanks in advance!