Obviously interest change over childhood years but, if I'm asked to select the one thing which I was mad about for longest, it's got to be Lego!
Like others here, I also enjoyed reading but simply going out with mates on our bikes, to explore all of the villages in our part of Suffolk was great fun too. (I find myself laughing when I hear of kids going on, say, a 10-mile sponsored cycle ride. My mates and I considered that anything less than 50 miles in a day didn't really count as a 'proper' cycle ride!)