To take constructing with Legos to another level, you might take a look at Lego Digital Designer. Yes, you can now design your own Lego creation ~and~ order it in kit form (be prepared for sticker shock). Beware, though, it's addicting.
My son enjoys designing ‘mini-kits’ using Lego Designer. He gets the part numbers for each piece from the lego website, and then we go to the Lego Store (there is one about 10 miles from our house) and place the order there instead of ordering online from home. It saves shipping fees and is much less expensive than ordering using the lego designer software. If you do not have a Lego store near you, you will still have to pay shipping, but ordering from the online store should still be less expensive than ordering from the lego design software.
The way it works is that you place the order, but before submitting it, you purchase a gift card for the total amount. Then when you submit the order, the store clerk overrides the shipping fee, and you pay using the gift card.