I would second @sufian00 here, because, its cold, otherwise Charna Island activity (Snorkling+Scuba Diving) would take good 5-7 hours plus hell lot of fun.
Though as friends are from Lahore, they could bear the Karachi-winter, so a night stay at nearby becahes (Hawksbay etc) would be a fun, make sure you hire some Makrani driver atleast
just to be at safe side.
If they are nature lover, Kalri Lake or Gorakh Hills might spice up the fun and will allow camping + day/night stay.
Breakfast at Boat Basin with Charsi/BBQ Paratha
Port Grand - Boat trip + Dinner will give another angle to fun and consume their 4-5 hours
My 2 cents!