The town is very much the holiday destination for Morocco's more wealthy city dwellers. With long beaches and a more relaxed atmosphere, it makes a welcome relief to the hustle-and-bustle of the larger cities inland. The town's other economy is its fishing industry.
The town is famous for the remains of Portuguese buildings and the Cisternbut these are fairly limited attractions. Even the most dedicated historian may find their interest waning after an hour.
There are many (3-5?) buses a day from Agadir to El Jadida. From the north, there are also buses (howmany??? 2?) a day all the way from Rabat, through Casablanca.
Many buses arrive at night. You are likely to find your hostel or campsite closed at 3:30am. If you have sleeping bags or a tent ground sheet, sleeping on the beach is possible. You may wake up damp from the salt spray though.
ONCF  trains depart nine times a day from Casablanca Voyageurs train station. In February 2018 the first departure was at 6:40 am, followed by departures at 8:40 am, 9:40 am, 10:40 am, 12:40 pm, 2:40 pm, 4:40 pm, 5:40 pm, and 7:40 pm. The cost for a round-trip ticket is 8 dirhams in second class, which is fine for nearly all travelers. The trip is around 1 hour, 30 minutes. Trains for El Jadida also depart from Casablanca Port, but they go through Casablanca Voyageurs and cost a bit more. Taxis from the train station in El Jadida to Cité Portugaise cost a flat rate of 10 dirhams but expect to share a taxi with people going in the same direction (but not necessarily to the same destination).
El Jadida is quite spread out along the sea front. It probably takes an hour to walk from one end of the centre to the other.
A few more 'western-style' restaurants along sea front, north of town centre.
Mint tea is available at the massive cafe in the centre of town, down the main road from near the entrance to the Medina.