Hoi An is a very nice city, very easy to visit by bike: explore the countryside, visit the market, go to the beaches... by yourself, just you and your bike.
MySon is a little bit further, so you'll need to take a bus or book a tour.
1. Bike ride in the country side
You can rent a bike everywhere (between 20 000 and 30 000 Dg/day) and visit the city and around by yourself (honnestly, no need to pay for a guide).
If you're not with a group it's easier to meet local people; they will invite you to see their house, drink rice alcohol.... You also can see some workers in coconut grove, painters of fishing boats, explore some villages along the river and make some really nice pictures.
Don't be scared to get lost!
If you need an exemple of a road, you can find it here:
Better to go there early in the morning (around 6 or 6.30).
Villages, fields and coconut grove
An Bang Beach
From 5pm some vietnamese come on the beach and cook some fish.
You also can enjoy a beautiful sunset (on the way back - from the beach to the city)
2. MySon santuaryYou can go there by yourself but it's easier if you book a tour.
Around 1 hour by bus to get there.