Quoting I'm His Amy He's My Rory:" We just have one blanket we use all the time for going out. It's bigger so it wraps around him well. ... [snip!] ... But other than that it's just a blanket wrapping him up. ETA: outside the car of course then covering him once he's buckled."
Quoting anonymom + 1.5:" I couldn't keep that on my 2.5 year old and carry the diaper bag and little guy's carseat, too... Maybe a poncho would work. Eek, I hate winter!"