We have ones that I made out of felt (got the idea years ago from Family Fun). Each day has a pocket and each pocket holds a treat. Usually it is a small piece of candy (until recently raided out of their trick or treat loot) or a little toy or gadget. When they were little it would be things like matchbox cars or little toys with Christmas themes. Now they get mostly candy, a few $1 bills, an occasional movie ticket, key chain. On the 24th they always get Christmas pjs (well now just the lounging pants). We've been doing this since my oldest (now almost 17) was about 3. They would be very disappointed if I stopped doing it.
When I was a kid we all had a Christmas countdown, also made of felt. There were 24 mini candycanes tied to it with green yarn. There was a bell at the bottom and a poem that said it would be Christmas by the time you reached the bell. I loved that thing and we were always counting how many candycanes were left because we couldn't wait for Christmas!