In Tunis you must visit the ancient ruins of Carthage which are close by.
If you are fit and healthy Corfu Old town is about a half hour walk from the port and is very nice....cobbled streets with pavement cafes and nice little shops..we left the ship there and as our flight wasn't until late in the evening we had a lovely wander through the streets there.
Malta we just got the bus from across the road into the bus station at Valetta and from there you could catch a bus anywhere on the island....or just wander Valetta itself as we did. It was our first visit and on first visits we always just explore the local town/city.!
In Malta take a taxi and go to Medina - in Tunis take an organised tour - in Corfu take the shuttle bus into the centre and just walk around the old town - never been to Almeria,sorr.
I live in Almeria and I think that the ship will probably doing tours to Granada or Sevelle quite a drive but very beautifull the Alhambra palace is magnificent as is sevelle Taxis etc do not speak an awfull lot of English her.