There's an excellent restaurant called Le Cous Cous situated around the fountain area. They do lovely cous cous with fish, vegetables, chicken, lamb and other meals.
Also for a nice lunch not too pricey is La Spada around the marina area.
Also there's a restaurant called Le Mediterranee around the marina that does lovely cous cous meals with music in the evenings.
We went on the submarine boat, we paid 20 dinars for 30 minutes by the marina and came back feeling absolutely sick, didn't see that many fish either. Ask to look inside the submarine boat, there were 2 and neither was very spacious inside, they fit approx 6 people seated max, never again for me, they will try to charge you 35 dinar, if you really want to go, insist on paying no more than 20 dinar with a free child place. Don't eat before or immediately after.
We visited the zoo on the marina, it cost 6 dinar for 1 adult and child together. Not much to see, but worth a short visit. They have silk hens, civilised rabbits, emu, ostrich, camel, budgies, other different birds, ducks, different kinds of hens tortoises. We never stayed long but it was nice, my 7 yr old was entertained and we were both glad we went.
If buying souvenirs, best to go to Soula centre in sousse at the mouth of the medina, check out the fixed prices and then go to PEK and haggle for prices below that. Chances are you will get everything cheaper in the medina, PEK is expensive for souvenirs.
Love the fountain with the seating area, the music, nice to sit there and be chilled out in the evening.