On a Friday night, get your teens together at the church building. You will need lots and lots of tape, staple guns, paint, and boxes of all sizes (from banana boxes to refrigerator boxes). You may want to make an announcement for members to donate boxes. Have them build a maze from the boxes. Depending on how big you make your maze, you may need two nights to do this. You can make any kind of structure(s) you want, such as a castle, village, etc. Have the teens decorate it any way they choose.
On Saturday, invite all the elementary and preschool kids to the building for a play day. Have the teens act as tour guides through the maze. Once all the kids have gotten to go through it, just let them play. This is a great event because it doubles as a play day for the little kids and a service project for the older ones.
Note: Make sure your structure is reinforced well – little kids can rough it up pretty quick!