Legos are timeless. I have a huge plastic container full from all the sets I bought for my sons...who is now 32!! They will go to his kids one day but right now they stay at Nana's house to play with when they come.
I gave my 8yo DGS a K'Nex set for his birthday in November. I have pics of his dad (my son) and him working on it. It goes together different from Legos. He loved it so much because it has actual moving parts so DDIL suggested I get another in the series for one of his Christmas presents.