Being a UNESCO World Heritage place, Hoi An's old town is veritably bursting with history. Of course, you can walk through the streets for free, but if you would like to visit several attractions in one day perhaps it's best to buy a Multi-entrance ticket.
You buy them in any of the ticket offices that are in some travel agencies in the old town. With one ticket (about US$3) you can visit one of the three museums, one of the three 'old houses' and one of the three assembly halls. The money goes to the old town maintenance.