Hi don't know anything really about Djerba it's self but as for excursions here is a link for Thomson's to give a rough guide of what there is to offer , But the hotel you staying at will probably have these excursions for maybe less money :)
As you probably aware in Tunisia it is a closed currency so you not ment to take Dinars out of the country oh and in , When I was there I still had some Dinars left (I forgot Lol) this was after security so I used them to buy something to eat/drink n some sweets , Problem was when I went to the duty free shop I went to buy cigarettes and they charged in Euro's , I had no Euro's so I paid using my debit card with no problems ! This was when I was returning home ! So from what I can remember you cant use Tunisian Dinars in the duty free shops ! Euro's only , So I guess your choice if you want to take some for duty free on your inbound flight !
If I am wrong with this info sure someone will correct me :)
Hope you have a great time !