Get creative. Find toys that give your baby a chance to expand on his skills. A container of blocks in varying shapes, colors, and sizes provides infinite learning possibilities. The same goes for nesting cups, which can be lined up as containers, turned upside down as towers, or used for water play.
Remember what he likes. If your son always smiles at the neighbor's puppy, get him a stuffed dog and help him make the connection that the toy in his room is a replica of the pooch outside. Give a budding engineer who likes the trucks on the highway his own set of rigs, and encourage him to mimic their sounds as he plays.
Consider going low-tech. Since babies quickly learn the association between pushing a button and the result, the novelty of an electronic toy will likely fade well before the batteries do. There's no reason to think that a super-duper "educational" toy is any better at entertaining your baby or gaining his later acceptance to an Ivy League school than a simple plastic bowl and some spoons.