ShaKEEn,

Mountain flight and rafting both safe.   Mountain flight for foreigner around $100 per person but you might get Nepali rate which is a lot less.  It is one hour flight and they won't fly if weather is bad.  Trisuli River rafting, $100 should be enough for a family of 4-6 (one raft for one day) and may be another $100 for private car or van.

Find some friends or family and go Fishling to Mugling or Mugling to Narayanghat in Trisuli river.    Call Kumar Rana (Nepal Outdoor Center) or Kumar Gurung (Teres Himalayan Adventrure) for rafting.  Their phone numbers should be on the river rafting agents list on Nepal Travel or some magagines.  Call Arun Dali in Himalayan Journeys for mountain flights. 

Have fun