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 Cistern. Although 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 every 2 hours from Casablanca Port and Casablanca Voyageurs train stations. The trip is around 1 hour.
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.
Relax in one of the restaurants or cafes in the town. Especially if this is towards the end of your trip, as it is for many. You'll find this town far more chilled than Marrakech or Fez
Tiny "Medina" not worth it.
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.
Campsite, The campsite is at the North East end of town, located about 5 minutes walk from the large roundabout near the sea (The Rough Guide is wrong). Price