I certainly wouldn't book with any agents. If you go to the town and visit Mandraki Harbour you'll be spoilt for choice with boat trips, much cheaper if you book them yourself.Most of the boats will sell you an open ticket for you to choose which day you want to go. They start quite early so with an open ticket if you oversleep you can go another day.Symi is a lovely trip, choose a boat that calls at Panormitis first so you can visit the monastery.There are also trips by boat to Nissiros and Thilos. A shorter trip by boat to Lindos is also worth doing as the buses get very crowded.A visit to Ancient Kamiros can be done by bus and is well worth the effort if you like ancient Greek history.There's also some delicious fish to be eaten in Kamiros Scala, at the bottom of the hill.A bus (or taxi if you want to cut out the long walk uphill) to Filerimos will take you to the monastery and to the huge cross. You can go up inside the cross after walking the Via Dolorosa and the views are spectacular. I could go on and on! Enjoy!

Agree with Anne - you are close enough to Rhodes Town with good public transport so organise your own. The best way to see the island is by car as buses and boats won't get you to all of the many sights worth seeing. If you're not into boat trips Rhodes Town, Filerimos, Ancient Kamiros and even Lindos are all easily accesible by bus/taxi.