I prefer the Heatsheets 2-person blanket -- it's still small enough to fit in very small kits, but it's a lot more sturdy than the cheap mylar emergency blankets.

Recently I added a small, inexpensive tarp to my car kit (http://www.rescue-essentials.com/base-all-purpose-tarp/, no affiliation) but I haven't had time to do anything with it yet. It's small and light enough to go in a backpack fairly easily.