For a younger kid, a ball is often a good option.
This Christmas, after putting aside the push car, some books, and a few other little toys from the grandparents, my 1 year old has spent the past 30 minutes chasing a large beach ball one of his siblings brought up from the basement.
I can second the recommendation for magnet tiles, though; everyone in the family seems to enjoy the satisfaction of them clicking together, and finding new ways to build random stuff. The toddler just makes stacks of magnet tiles, which is fine for his development. The 8-12 year olds enjoy building relatively complex structures. Then watching the 1 year old act like Godzilla an destroy it.
Blocks is the top comment (for me); and yours is number two. Timeless classic. Another one could be plasticine clay. These toys afford play, they don't direct, restrict, or guide play. Other good toys like this: box, stick, the woods, paper (especially a big roll of butcher paper), and things to draw with (I find a black, red, blue, yellow and green sufficient).
For all the toys my kids received until they hit 3+, they probably got the most enjoyment out of cardboard boxes.