For Duplos, I highly recommend the Duplo Educational line - much more creative than the dinky sets in regular mortar and online stores. My 8 yo will still find ways to play with them, so a good investment. We got a lot of the special pieces we wanted through Ebay.
The giant wooden marble runs were also favorites here and I second that when they're older the plastic are much, much easier than the wooden block style. Plus, you can get four sets for much less than the price of a small wooden set and they can be super, super creative with that.
For expensive wooden toys, we have gotten tons of use out of Haba's Sticki Steine. they are wooden blocks, but have bumps and grooves like LEGO. We found several used and have a very nice, very large set (maybe 110 pieces?). Every age visitor loves them.
For gross motor, we got tons and tons of play out of the BIG Step 2 slide, the Up and Down Coaster and the door-jam swing and bars. My kids could both ride the Up and Down Coaster without help by 8 months (they were walking at 8 and 6 months, respectively) and could climb up and slide down the tall slide without assistance when my oldest got it at 12 months and the younger was off on it at 5 months with us catching her. They still use it. We keep it indoors. You can roll colorful wiffle balls up and down the Up and Down Coaster or the Slide, too, then gather them up into a bin or sort them and do it again. Oh, and that Cozy Coupe thing got a lot of use, too. We got ours on freecycled.
We also have a huge set of the 1972 Little People. My husband and I and most of our friends agree they are much more conducive to creative play than the new ones. They are also pretty easy to find used. Right now many of the pieces are being used in an elaborate scenario envolving wooden trains, wooden cars, wooden people, blocks, LEGO, fabric, playsilks, cotton, Schleich animals, the doll house, a FP barn, FIMO creations and PLAN roads. These toys grow and grow.
I also recommend the following board games. My kids were playing them easily at 12 months. HABA: Orchard, The Snowman Game. Selecta: Corso Canneloni, Piccobello, Duomino, Grab It, Memo Primo.