There are day trips to Isabela, the ferry will take 2 hours to get there and 2 hours to get back. These tours usually visit Tintoreras, the flamingos lagoon and the breeding center. A reliable company is Book ec.
I stand corrected - though with such little time in the islands, I'm not sure that spending that much of it "commuting" to other inhabited islands is the best use of it. You may also want to read about the discomforts of the speedboat ferries between the islands before deciding to spend 4+ hours on them / day. I lived on Santa Cruz for 4 years and you can easily find plenty of fascinating things to do with that as your home base.