One idea I came up with was a magnetic marble run. I saved a bunch of paper towel and toilet paper tubes and when I had plenty, I attached some dot magnets and cut semicircles at some of the ends. I had all these visions of big elaborate marble tracks crisscrossing my fridge. I saved a yogurt container that I planned to use to "catch" the marble at the end. Xander was super excited.
Sadly, it was not to be. The magnets weren't strong enough and the marbles were too heavy. When they would come to an end where we expected it to go back the other way, the marble would rotate the tube and fall to the floor. The magnets also fell off the yogurt container. We were both a little disappointed. However, we kept playing with it and found that Xander liked making shorter tracks and shooting them into a plastic shoe-box that I put all the tubes in. It seems to work fine as long as you don't try to switch back the opposite direction down the fridge. That first day he played almost half an hour (eons in toddler time!). He's also gotten it out since. Oh, and used the tubes as telescopes, microphones, swords etc.
Even though it didn't quite turn out like planned, it still ended up being a fun, creative, inexpensive activity for him. The magnetic tubes will probably stay on our toy shelves for quite a while.
As happy as I am with this homemade version, I will admit, I purchased him some transparent marble runs from Lakeshore because they're currently a bonus buy. I'm excited that we'll have both kinds of runs to play with!