Karnala is on the Mumbai-Goa highway, NH 17. If you wish to travel by train, Panvel is 12 km away. Autos are available from the station. The trip will cost about Rs. 80-100.
Karnala has a small bird sanctuary, about 4 sq. km. Visitors are allowed in the mornings and evenings. Carry a camera to shoot the birds.
Karnala Fort (also called Funnel Hill) is within Karnala Bird Sanctuary. It is at a height of 1400 feet. You can trek to the top. There are two routes, one easy and the other difficult. You will see many birds and greenery. Watch out for bee hives.
It is good for a one or two day trip. Not much shopping to do. There is a well-known nursery if you like gardening: Go Green Nursery.
I stayed at Shalom Resort, phone: [--]. It has a swimming pool, gardens and a restaurant. There are 2-3 other places to stay too.