Bangalore-Madikeri-Kasargod-Mangalore-Dharmastala-Bangalore will be a good round trip for you to cover in 3 days,Many treks,waterfalls,beach,mountains, wildlife safari are included in the route
Then head toward Kodaikanal, it's a beautiful place i went there and also know for its magic mushrooms. You can also go to Chikmagalur nearest to Bangalore