You can go to Mangla Dam Boat club (Operated by Pak Army). Its much before Mirpur city. Here you can enjoy BBQ beside the lake. Another option is go to Mirpur and then go ahead of Mirpur city towards road that lead to Kotli. Here is excellent bank of Mangla Lake. Pic Below:
Or if you want some off the beaten path place, Checkout my thread.. its interesting place.. we had BBQ there..
https://www.pakwheels.com/forums/road-trips-vacations/208973-trip-nala-ghan-nala-kahan-malot-village-jhelum-district
Its on back side of Rohtas Fort on Jhelum-Pind dadan khan road.