The best toy my sister and I got is a now long-discontinued product called Omagles. They were brightly colored tubes and panels you connected with plastic clips. They were strong enough to stand and climb on. They even had wheels we could make vehicles out of.

They were so good I bought a used one for my own kid who had a great time with them.

After some Googling, I see that the rights to Omagles were bought and are now sold under the brand Tubelox.