The big favorites here, when I had SA kids in the summer, was the zip line, tree house, trampoline, calligraphy sets, laser tag, water balloons, air dry clay and water sprinklers.
Now, I did the unpopular thing of allowing them to get on the playstation, computers and game boys during nap times for the youngers. It was a hail Mary pass until the end of summer to keep my word to the parents. That was my last group.
IMHO Mixed ages like 11yrs and 3mnths are just too hard to keep everyone stimulated and active for 50+ hours a week. Especially without breaks WHILE also doing all the cooking, cleaning, refereeing, being the No-PDA Nazi and keeping up with diaper changes. UNCLE!!
(little chuckle that I can say Nazi but not c a s i n o
)