Is this for your own son? Or is it for a friend or relative?
If you want to keep costs down, any developmental toy 12-18 months is great, you can find many things between $10-$30 made by Fisher Price, V-Tech,etc. at any toystore or department store. Push toys (like carts, walkers, lawn mowers, corn poppers) are great too, as by this point they will be early walkers or learning to walk.
For more expensive gifts - Cozy Coupes, Toy Boxes, Wagons, small tri-cycles are all awesome all for different reasons. On a side tip - If for your own son or someone your close to, if you don't mind used, check out freecycle.com, craigslist, and local lawn and garage sales. You'd be amazed at what you can find. For our son we were able to find two used Cozy Coupes, a Little Tikes Playhouse, and Little Tikes Toddler Slide all for Free (and they normally cost hundreds of dollars new!)! And another secret... he dosen't care that they were used! He loves his cars and house, and has no concern thet the plastic might be faded or some of the stickers are gone.