Hmm…I don’t know about 18 months. That is such a tricky age to entertain LOL. I totally understand.
My youngest is 3 now and one activity we do together are giant floor puzzles of the alphabet or maps. Mostly alphabet/numbers. We sing an ABC or 123 song and make kind of a race to put it together…if that makes sense?
Another thing we do is to “fish” for words. I cut out construction paper fish, write a word on it and attach a big paper clip to each. Then I use a cheapy fishing game rod I found at a dollar store for the kids “catch” the fish. I have my oldest make sentences out of what he catches and my youngest just gets a kick out of getting them on the pole.
Also, I’d check out the Cranium games and http://www.first-school.ws/ (and websites like that). We also have a store near here called Teaching Stuff ..they’re online at http://www.teachingstuff.com/ .
I plan on getting a giant piece of felt and putting it up on the back of our “school room” (converted the study to a classroom of sorts) door. Then I’m going to get the felt cut outs for felt-boards so that my youngest can make up stories/scenes using the felt and I’ll get the felt anatomy set to help my eldest learn more about the body/biology.
Hope that helps! I didn’t do much with my son when he was 3-4 other than the usual: reading, games, crafts. When he was about 4.5 I started getting him to sit down and do phonics for 20 minutes or so at a time and then I added some worksheets–but not many. You’re right, before 6 years of age a lot of the learning is through play, etc…not so much “formal”/”sitting at a desk” learning.