Here is a photo of a crane we built using a load of 2x8 and 2x6 beams and 2x2 and 1x1 stud bricks for a tower and a massive block of yellow bricks to balance out the jib. While my son did enjoy helping to get something built that was taller than him, the real fun for him was knocking it over and watching hundreds of bricks fly everywhere! Ho hum.
The Lego City kits look really exciting and promise a lot of building and playing fun, but I still think the kids get more fun when I buy them big buckets of hundreds of bricks and order loads of beams and plates, wheels and window bricks from the Lego website. David can spend hours just building loads of weird and wonderful cars and lorries from his imagination rather than following instructions from a booklet; it's far more creative.
Anyhow, rainy weekends let me get the Lego out and regress back to being a kid again - and also help teach my son the tried and tested techniques of building fun towers and buildings in the process.