Playing "slurpies and burpies" (figure it out- they slurp the Coke and try to burp as loud as possible)... One of the silly games we played.
We didn't only play games and swim. Worship and Bible study times were precious, with Francis Angalla leading worship, Brent Duncan sharing in the evenings, and Shanna Sanders sharing for the morning devotional times. 
Hanging out with friends is a key ingredient for retreats.
"Bobbing for Donuts", a part of the relay we played one day.
We have 4 little platform trolleys. Add plungers and a teen, and ask them to paddle down to a line... not as "easy" as it looks. Lydia here (a senior from the USA and Uganda) demonstrates how easily the plungers got their suction going...