Is this possible to do for a day trip? What is the best means of transportation? I'm thinking we will need to get to Split.
Day trip would be possible but hard to make. Take earliest ferry to Split first, catch 8.30 am bus on the way to Zagreb but the one taking route on old motorway, not highway and you will be there around 12.30 pm. Return could be arranged by buses leaving later evening or night arriving to Split early in the morning next day. I think better option is a day trip from Split made by some of local travel companies.