One of our all-time favourite toys would have to be our Marble Run. I remembered the kids at kinder absolutely loving this toy, so I bought one for Em (well, Santa delivered one) when she was around 2.
She has had hours of fun with it and in the past year or so Ed has started to really enjoy it too.
It is something that requires a reasonable amount of adult-help to begin with. I've taken the time to build some runs with the kids so that get an idea about how it works.
But, once they get the hang of how it works, it allows for lots of opportunities to problem-solve, plan, build, co-operate, negotiate, follow instructions, imagine and create on your own, hypothesise and then of course, there's the irresistible fun of sending lots and lots of marbles scooting down the run.
If you don't have one at your house, I highly recommend it. It's worth investing a bit of money on one with lots of pieces and sturdy construction and buy an extra bag of marbles for even more fun!