OK, I told you I wouldn't go crazy with the kid stuff, but we all know some of us are "old" and have "children" and need to "watch them" sometimes. So, in the interests of fostering familial harmony I give you... the Chariot Carriers Cougar 1 stroller.
Calling this thing a stroller is like calling Realization a good weekend project. The 1 is for 1 child; there's also a Cougar 2 for... well, figure it out. The Cougar has extremely low rolling resistance, super smooth adjustable suspension, five point safety harness, roll cage, plenty of storage, wind and mesh roll-up covers, padded handlebars and a parking brake. It folds up a little bit to fit into a trunk even.
What makes this an awesome cross-training device are the add-on kits, at least one of which must be purchased to make it useful. There are kits for biking, hiking, strolling, jogging and skiing. I've used them all except the stroller (oh the irony!) and they all work as advertised. The ski system works for both skating and classic styles, with skating being a little smoother, as well as with snowshoes. The jogging system adds a front brake which makes it useful for rollerblading downhill.
Works up to 75 pounds, so it won't be obsolete after the first growth spurt. This thing is seriously well-designed and worth its weight in gold. There's nothing like the look on a one-year old's face as he sits in the cockpit at speed while you push him down the trail!