Here's the recipe per pair of campers:
1 cup milk
1/4 cup sugar
1/4 teaspoon flavoring
1/2 cup rock salt
2 cups ice
1 quart-size freezer bag
1 gallon-size freezer bag
4 full sheets newspaper
1 strip of duct tape
(I am reversing the assembly line this year, so instead of the counselors delivering the ingredients to each pair of campers, the pairs will go around to stations to receive their stuff.)
Table 1: No counselor needed. Pairs pick up one of each type of freezer bag.
Table 2: One counselor pours milk into quart-size bag. One counselor pours ice into gallon-size bag.
Table 3: One counselor adds sugar to milk. One counselor adds salt to ice.
Table 4: One or more counselors add desired flavor to milk and sugar. (I'm using baking extracts from http://www.spicebarn.com/. The same extracts are used in a scent sense game earlier in the day. The extracts relate back to the idea that plants are food sources for people. We have peach, lemon, blackberry, spearmint, and coconut, because they are all clear and cannot be identified by color - which would spoil the scent game.)
Table 5: One counselor inserts milk bag into ice bag, seals outer bag, and passes to one counselor who wraps the bag in newspaper like a Subway sandwich (I knew that job experience would come in handy somewhere), and passes to one counselor who tapes the package. (These tasks can be done by one person if there are not enough instructors, counselors, or chaperones to fill all the stations.)
The pairs shake their packages vigorously, trading off as one tires. After 10 minutes, campers can unwrap the packages, extract the ice cream bag from the saltwater bag, and split the portion into two bowls.
There are science lessons that accompany this activity at teachnet.com, and about.com: chemistry, if you need to justify making ice cream.
Here's something I think you might find useful:
Regular bags of granulated sugar are 4 pounds, which works out to about 9 1/2 cups, so each bag serves 19 pairs of campers.
Regular boxes of rock salt weigh 4 pounds. 4 pounds of rock salt = 6 1/2 cups, so each box serves 13 pairs of campers.
You can find both granulated sugar and rock salt in larger packages, and buying in bulk is usually more economical. Remember, y'all aren't eating the salt. It isn't actually coming into contact with food, so it doesn't really have to be "ice cream" rock salt. The dirty-looking street salt works just as well, and is even cheaper.
I'll have to post a follow-up to let you know whether or not the campers liked peach, lemon, blackberry, spearmint, and coconut flavored ice creams. (I do have vanilla extract in reserve just in case).