So, we've found that the well-reviewed Graco Snugride car seat fits right into the also well-reviewed and very lightweight Peg Perego Aria stroller, helped along by a couple of easily-formed nylon straps. (Buy the straps at Narain's Outdoor Fabric in Berkeley, or at your favorite backpack repair shop.) The whole setup weighs only about 16 pounds (Seat + Stroller, no baby) -- No need to buy a universal stroller frame, the bulky Graco traveler system (35+ pounds!), or the not-as-well-reviewed (and expensive) Peg Perego car seats!
(Disclaimer: Your mileage may vary. We learned this by trial-and-error, and there are probably many ways to achieve the same result. I think it works well for our baby, and have loved this setup for the past month, but don't come after me if you try this and the chair falls out with the baby inside, in front of an approaching train.)
1. 5-6 feet of 3/4-in nylon strap webbing
2. 1 quick-release side strap buckle
3. 2 ladderlock buckles
On the Aria stroller, pull back the cloth seatback and find two pre-existing nylon strap loops. Loop your nylon webbing there, and close the loop using the quick-release buckle and a ladderlock buckle on each side. To keep the webbing ends from fraying, melt the ends with a lighter flame:
Recline the Aria's seatback all the way, and place the Snugride into the seating area, facing backwards. The red clip should latch right onto the snack tray:
Route the straps through the perfectly-placed hook on the Snugride, and you're done! The setup is light, maneuverable, sturdy (so far for us -- I've tipped it to pretty severe angles and haven't gotten anyone hurt), and is all you need in place of a travel system!
I've tied the excess strapping into a knot, but should probably just cut it off...