TOURS vs DIY HOI AN?
Sites around central Hoi An is easily seen on your own, and you can arrange boat trips from the docks in the center. If you don’t want to bike or motorbike around, taxis or motorbike guys will take you to the beach a few kilometers east.
After that you’ll hear offers from travel agents and guesthouses for group tours to nearby My Son, snorkel/diving trips to Cham Island, or guided trips to Danang’s sights (the Cham Museum and Marble Mountains). Presently no regular group tours go to the My Lai site, a couple hours south — but you can arrange private tours from many travel agents.
The most useful tours are to Cham Island (which is hard/impossible to do on the local ferry) and to My Son (particularly the ’sunrise’ option when few visitors are there). If you motorbike, you can easily get to Danang on your own — just follow the beach road north towards China Beach and the Marble Mountains.