Some areas of Fort Cornwallis are undergoing archeological excavation and restoration but you are still able to walk along most of the grassy wall embankments with their cannons. That’s really all there is with the area inside the walls basically a park. You can see the signal tower (though not access it), munitions bunker, a later-day statue of Captain Light and some cheesy cut-out face props for photos. You can get a higher view of the Queen Victoria Memorial Clock Tower from the ramparts.
We happened to come just before a free tour started. Molly was our guide and gave us some interesting background to the history of the fort and Penang itself. Molly was approachable, entertaining and conveyed the historical importance of the fort. Without her I am not sure we would have found the visit overly worthwhile. There is a nice gift shop, clean toilets and cafe where we had a drink in the shade. There was supposed to be free wifi but we weren’t able to access it.