Swimming would be the main objective going there. Barbeque would be fun too but carrying the barbeque pit and the rest of the steep slope then carrying the up when going home - think twice but if you have the people, the energy and the will, by all means go ahead - adds to the fun!
Otherwise, carry with you some nasi lemak bungkus or some home cooked meal to satisfy your hunger after a dip in the cold waters.
Be sure, whatever you eat - carry the rubbish with you when you leave.
be careful of some parts where the waters are deep - i heard quite a number of people already drowned here.
On your way back, stop at Batang Kali town to enjoy the Mamak Cendol there. It'll be worth the stop.
If you have weak knees or haven't exercised for a very long time - think twice before going hehehehe.
be extra carefull when swimming as some parts of the waters are very deep and beware of undercurrents.