Penang is a small island so it's more convenient if you could rent a car. Would highly recommend Georgetown (now declared a World Heritage Site), try the new tram at Penang Hill (Bukit Bendera). Penang Butterfly Farm is great if you have kids.

People visit Penang for its world famous food, commercial beaches, heritage and melting pot culture. If you are up to it, break away from conventions and 'take a hike' at Penang National Park, situated at Teluk Bahang. It is next to a fishing village and you can even ask the boat man to organise a fresh seafood barbeque lunch at Monkey Beach . Not many people know about this tropical jungle attraction.

The national park is also the home of one of only two meromictic lakes in the world. It contains 2 layers of water (warm and cold) that never mix. You can also see baby turtles and hike to a lighthouse. Found a link on the internet providing good insights to Penang National Park here - http://www.malaysia-tourism-guide.com/penang-national-park.html

You have quite a number of options with 3 days on your side. Suggest you explore Georgetown (now a World Heritage Site), Penang Butterfly Farm, Batu Ferringgi beach. Penang Snake Temple is not quite as intriguing as in the past but still a good stopover. Development in surrounding areas must have disturbed the snakes' habitat, reducing the numbers significantly.

Here is a useful resource I've used during my last visit to Penang - http://bit.ly/ewFoVA .

Have a good holiday