1. Penang Botanic Garden is more of a hiking heaven for Penangnites than your usual park. If you love hiking, that is the place for you. Else, yes there is nothing much to offer. If you know the tracks, you'll be surprised to where the trails might bring you. You might be interested to know that on weekends, there are loads of people over there.
The Botanic Garden is generally a starting point for seasoned hikers before they walk over the Penang Hill or other parts of the island. I didn't think much of that place as well, till my gf's father brought me along one of his hiking trips.
2. Speaking of food, I guess you hv to get a local to bring you around. Some of the best seafood is way up north and it is a good 1.5hr drive from Georgetown. The best street shops are scattered all over Georgetown streets and they are opened only at night. Even the hawkers near the jetty are hidden away by a huge tree!
Penangnites don't go to G Drive or even Burma Road for their laksa fix for a start.
I'm sad to hear about your trip, but sometimes in the future, when you know a local, give yourself the opportunity to go over again. (Though yes, I have to admit the beaches are going down under...)