D.W.
If the weather's nice, I'd recommend spending as much time as possible outdoors - park, sandbox, story time at the library, walks through the neighborhood talking about things they see, etc.
Sure there are workbooks you can purchase......if you can find a school supply store, you're likely to find laminated posters with shapes, colors, numbers, alphabet, maps, etc. that they can work on.
At 18 months, a piece of paper, some crayons and some stickers can be hours of fun. So can a bouncy ball for $0.25 from Old Navy, some metal pots to bang on, etc.
Our kids are in day care.....we had a sitter come to our house until our son was 21 months and opted to go with conventional day care after that. It made a huge difference as day care has a structured curriculum and devoted instruction time each day. Our pediatrician said our daughter is about 6 months advanced on her developmental milestones - while we reinforce things at home (colors, counting, etc), the basis of the instruction has come from day care.