The island is lovely for a trip. The ferry from Dubrovnik old port is only 15mins and goes every half hour. Most of the cost is paying admission/entrance to the island, and you can see what you pay for; well maintained amenities, first aid posts, and signs.
The views from the old fort are spectacular, though it's a bit of a hike up to it, I managed in walking sandals. The botanical gardens are nice too. Plenty of places to eat and drinks (restaurant Lacroma was fantastic - desserts were lovely)
The beach looked nice, though we didn't use it. There is also a play park for kids.
The semi-tame rabbits and peacocks were everywhere! So we're the signs not to feed them, but no body was paying attention to those
Ferry was a little busy leaving but since they run till 7pm at this time of year it's fine.