If there is one toy that my son plays with more than any other it would be stacking cups and stacking rings. Why kids love them so much I don't know, nor do I know why I'm so interested in them either. They are well worth the minor investment and great at helping to develop motor skills. If you didn't have these as a child you are probably in counseling because of it.
Every kid loves to build something for the sake of knocking it over and starting from scratch.
Again we are talking motor skills, as well as some imagination at work. I would recommend wooden blocks with non-toxic paint, and if you're lucky you won't have to take out a mortgage to afford them. A whisk and other utensils. A quick trip to the local Salvation Army or yard sale can turn up a gold mine of used kitchen utensils that will provide hours of entertainment for your infant at a fraction of the cost of real toys. This is an economical, recyclable jungle gym.