I think that's about the age when my DS became obsessed with wooden trains. Or rather, the age when his prior obsession was allowed to develop at home because he got some of his own and wasn't only able to play with them at Barnes and Noble. They've been really wonderful for him through recent developmental leaps. When we got him his first set three months ago, I thought maybe he was way too young and wouldn't be able to manage even to move them around on the tracks for many more months. Now he has magnetic polarity figured out and is also putting the tracks together himself. And some days he's so engrossed in them that I can actually get some cleaning done while he's awake.
North Star Toys has WONDERFUL handmade wooden toys for very reasonable prices. Their service is truly amazing, too...like, a personal email to apologize for a shipping delay I never would have noticed while they were visiting family and then a handwritten card in with the toys when they came. The Rainbow People Mover is really cool. DS also plays with the wooden camera I got him from there. I would love to get him more of their toys. http://www.northstartoys.com/collections/all-collections/
Plan Toys also has some good ones. We have two of these zebras, and he loves chasing me while I pull both of them, and started pulling them himself at around 15-16 months old. http://www.amazon.com/Plan-Toys-0514600-PlanToys-Pull-Along/dp/B0015D47WS/
Duplo blocks aren't natural/wooden, but they're another thing we all play with a lot that I really like the way he plays with them. Otherwise, books and musical instruments. You can pick up a good metal harmonica on Amazon pretty inexpensively. DS loves playing his (DH got two of his own after I got a child's one for DS, so they play together...and I have no idea where that child's one is anymore), and it's amazing how much it sounds like music, especially on a real rather than toy harmonica. We've also gotten into wooden puzzles in the last couple of months. Oh, and a play kitchen and tea set have both been big hits for months.
15-19 months has been such a fun age for play...enjoy! :)