Well, I personally, have always wanted one of those Plan cities!
I love that stuff and you can add onto it.
Legos- those are another one of my favorites and something Grandma could find at Target.
Good art stuff! An easel + paints.
I noticed a cool 'fort' thing somewhere this season. It was 'Waldorf'ish in that it was wooden panels with material in the middle of the panel. Then the corners of the panels fit together. Oh, it was in one of my Waldorf books, if you are a DIY-er at all.