The Castle closes at 5:00, and if you want to wait out the tides so you can experience walking the causeway rather than taking the boat, you may have time to kill after that 5:00 closing. (The garden is also closed.) Are there places to sit or things to see while you wait for the low tide?
I guess it depends how much time it will be until the causeway opens (half an hour? three hours?) and what you like to do. We were just there last week, and walked the causeway both directions after hours. We spent about 45 minutes poking around the parts of the island that were open, looking for footprints (Queen Elizabeth, Queen Victoria, etc.), walking out on the harbor wall, watching the sailboats and the waves, the view of Penzance and Marazion, enjoying the peace and quiet ... I just looked over the photos I took, and there don't seem to be any benches.