You can hire bikes at the AGN office near the postal office. The prices when we last used them were around 15 Euros per day. The resort is bicycle friendly although there are not that many places to cycle around. If you are based on the camp site though (especially if you are situated at the far end of the site), a bicycle can be great for getting around the camp site. Many people though tend to hire bicycles if they plan to go outside of the resort to the textile areas.
The best option though if you are camping is to bring your own bicycle and many people do this. having a bicycle is a great way to go outside of the resort for cycking to the main hyper-market in Agde and for getting to the main marina area in the town.