A lot depends on what you want to do.
A good option might be Quadro.
It's like building with pvc pipes but easier.
You can build something to climb on and when you get tired of that, then build something else with it.
It's good inside, and outside.
My mom got a builders kit for our son when he was little.
We built things for him at first, then he helped us, and soon he could build on his own - with supervision - you have to make sure it's a stable structure and all the connectors are properly fastened.
He so loves it!
He's in college now and still builds with it - tables, chairs, a stand for his archery target, etc.
In between uses - it packs down into it's box and can be stored out of the way.
It's impossible to outgrow this!
http://www.phunzone.com/Quadro%20Residential/index.htm