El-Jadida is a costal city, which is much more popular with Moroccan tourists than it is with foreign tourists. Because there aren't tons of foreign tourists the city is less 'touristy' in the sense that there aren't stalls everywhere selling the same goods, and the locals are welcoming in a genuine way and aren't trying to sell you something,
The main attraction in the city is the 'Cite Portugaise' which was built by the Portuguese as a fort to protect their port, and was called Mazagan by the Portuguese. The 'Cite Portugaise' is a UNESCO World Heritage Site, and is really interesting to walk around. There are two abandoned churches, and abandoned synagogue (all closed), and the main lighthouse of the fortified city has been converted into a minaret.
Because El-Jadida is a port town, there's a lot of fish to be found. Fish of every kind, cooked in every way possible. Fish tajine, cumin spiced sardine balls in bread, fried fish. I tried a few of the types of fish, and from the photo you can see that one night I got more than my dose of Omega-3 Acids (this dinner was battered and fried sardine, calamari, and sole).
There's a nice beach in El-Jadida, which is the other main attraction. The sand is really nice, and the beach pretty clean too. And of course they had the obligatory camel and/or horse on the beach that you could take for a ride for a minimal fee.