Djerba is lush!
I always come on these threads to mount a bit of a defence of Tunisia as we lived there for a number of years and I don't recognise the hell-hole other posters talk about.
It's a developing country in the middle of an economic crisis, and it's a Muslim majority country, although pretty liberal and relaxed by regional standards (separation of religion and state as well). So yes, a lot of the town outskirts are scruffy and full of litter, but the tourist beaches are generally totally immaculate and the water quality is brilliant. Some of the local wine is good and as long as you haven't got the cheapest hotel going, that will probably be fine too. Jamie Oliver had an episode in Tunisia in his most recent series, he went nuts for the food. The fish is great, usually grilled and very fresh.
It genuinely has some of the best roman sites in the world, and they are usually empty and are also dead cheap to get into. Do try and visit El Jem (the amphitheatre, also has a cool little museum) and Dougga is an amazing preserved roman town in a stunning location. You can have lots of fun in the desert (although best done over a few days), especially if you're a Star Wars fan. The roads are pretty good and most drivers are chill outside of the big cities.
The 'markets' in tourist resorts are pretty hassley. But also- it's a market, people go there to buy things, if you don't want to buy things, don't go. If you want to look at handicrafts there are good ones in museums or see if you can find someone to take you where the locals buy crafts and household goods. The ones in the tourist markets are usually a load of tat and overpriced anyway. And don't walk around town in 'beach clothes'- even in Tunis, which is pretty cosmopolitan, I wouldn't wear shorts off the beach. Maybe to a club.
I think it's also worth noting that lots of middle aged to older European women go to Tunisia to pick up younger men. So in some cases that market for men approaching you has been created in some tourist resorts. Do you speak any French? Younger people speak good English too but a choice swear in French usually works to get someone annoying to bugger off.
I am sure you'll have a lovely time, although it will be hot! I've got loads of lovely Tunisian friends and it's always sad when people diss the whole country. It's a very complex place, post revolution, but the history is fascinating. There are some fab independent hotels and guest houses as well as that part of the sector starts to grow.