From Lucerne it is possible to travel to Cres by buy all year around, to travel to Cres, you first have to take the bus from Lucerne to Rijeka, which runs daily during peak summer season, and several times of week, rest of the year, you can see details below. From Rijeka you can then take regional bus to Cres (Timetable you can see here). Total travel time from Lucerne to Cres, should be about 14-15 hours. If you plan to return to Lucerne, you should buy both bus tickets as discounted return tickets.