Last summer we bought an ice cream ball.  It is a small plastic ball with a metal insert to make ice cream.  There is space between the metal insert and the plastic to add ice cream salt (ice cream salt should be available at your local grocery store).  The ice cream ball churns and freezes the ice cream with a little help from the kids.  Ice cream balls can be purchased from LL Bean in pint and quart sizes.  We were fortunate enough to find one on clearance at Wal-Mart.  We discovered the pint size is the perfect size for two kids with a little left over. 

We have made ice cream in the house, just rolling the ball around on the carpet.  We have also made ice cream outside in the grass.  I wouldn't recommend rolling the ball on concrete or other hard surface, as it may crack the plastic.  You can eat the ice cream right away for a soft serve consistency or freeze for a couple of hours for a harder texture.  There is always the option of using mix-ins (like Coldstone Creamery) to stretch the amount of ice cream.

It is also a nice activity for a party.  You can have the kids roll the ball, make the ice cream and have a small taste at the end.  Be sure to buy extra ice cream (even the quart size won't be enough for a party).  Ask the kids to compare the taste of the homemade ice cream to the store bought.  What ice cream do they like better? 

If you are really ambitious and have an afternoon with the kids you may want to consider making you own ice cream cones.  The Ultimate Ice Cream Book by Bruce Weinstein has recipes for sugar cones and pizelle waffle cones.  Just think how proud the kids will be after dinner sitting down and eating both ice cream and ice cream cones they made themselves.