Before hand you will need to collect a bunch of rocks (all sizes and shapes, but best if the rocks will fit into girls’ purses & guys pockets). If you really want effect, get pretty big rocks that are kind of heavy.
The week before, give each person a rock (I had each one write their name with permanent marker on the rock). Ask them to carry the rock around for a week; to school, at home, etc. The next week, take them to a river, lake, pond, etc. and talk to them about how we have to let things go. In the past they may have had people or situations that have hurt them or angered them. The weight of these things become heavy (like the rock) and they have to let it go but they can’t pick it back up again. When they feel they are ready to “let go” have them throw their rock into the lake (as far as they can from shore).