My kids love their folders with all the cards they receive. From time to time they look at them, remember who sent/ gave them (which is important when half of your family lives in other countries), and it also gives an incentive to talk about other cultures, customs and countries. When a friend has sent a card from holidays, it often ends up with us taking the globe lamp out and checking the locations. It must help that I love maps myself.
I suppose this would be the method I would use to teach toddlers geography if I would be homeschooling toddlers? It is simple, cheap, educational and fun.
Now I still have heaps of stamps I collected as a child. Any good tips on what fun we could do with them?