Get some huge tube socks and fill them about 2/3 full of white rice. You can microwave them for a minute each then put them in a container to keep them warm, like a shiny fold up carry style cooler. The shiny silver lining will reflect the heat and keep them really warm. I use them like this when we are at soccer games.
I have a blanket I carry in the van and then when we get there I get all snugly with the blanket and put a rice bag in the small of my back, around my neck, even under my arms. These are places the blood is close to the surface. If the blood is warm the whole body feels warmer too.
Also, there are reusable hand warmers that fit the bill too. Just snap them to create the chemical mixing, place them at specific places on the body, then when you are done you follow the instructions and they go back to their original state.
Here are some links:
These are boiled to make them go back to the original state.
http://www.thewarmingstore.com/wonder-warmers-reusable-ha...
This site has 4 pages of various warmers, some snap styles, some battery operated, some disposable.
http://preparedness.com/inreushanbod.html