Tunisia is a Muslim country, and its culture and customs should be respected. Dress code is important, particularly for females. Whilst a lot of skin (even topless) is tolerated on beaches and within hotel complexes, a modest amount of exposed skin may be frowned upon outside these areas.
Be aware that the further south one travels, the more conservative Tunisia becomes. While most women wear western clothes in the Capital (which has a mix of Mediterranean, European and Arabic cultures), the south of Tunisia is practically devoid of any lingering European influence and is thus far more traditional.
Tunisia and Ramadan
Be prepared for a somewhat unique experience if you choose to visit Tunisia during Ramadan. During the daylight hours, eat and drink (even water), very discretely. Buy bread and focaccia from the street vendors in the evening for lunch the next day, or find one of the local shops that are still open to buy something to tide you over.
Virtually no one drinks alcohol, and your best bet (at least in Tunis) might be the Hotel Africa.