Hi Trevor,
A few suggestions for your cruise:
Barcelona;
the Gothic Quarter
http://en.wikipedia.org/wiki/Gothic_Quarter,_Barcelona
the Sagrada Familia
http://en.wikipedia.org/wiki/Sagrada_Fam%C3%ADlia
Las Ramblas
http://en.wikipedia.org/wiki/La_Rambla,_Barcelona
If you are a soccer fan, Barcelona run tours round their stadium (Camp Nou
http://en.wikipedia.org/wiki/Camp_Nou)
Rome: so much to see; possibly best to join a tour depending on the amount of time you have in Rome. If you do have the time, it's a great place to wander round. Presumabley the ship is berthing in Civitavecchia; if so be prepared for a bit of a journey into Rome (about 70km)
Naples: If you have time try a pizza in one of the restaurants that lie off the beaten track. And watch the traffic; absolute loonies!!!
Istanbul;
The Grand Bazaar - watch out for forgeries; prepare to haggle and to drink lots of apple tea.
http://en.wikipedia.org/wiki/Grand_Bazaar,_Istanbul
If you have time take a ferry across Bosphorus and visit the Asian continent!
Can't offer advice on a daily allowance but just be aware that most cruise ships run a cashless system where you pay at the end of the cruise. It has caught some people out. Internet is generally available on the ships but can be very slow (think dial-up rates) and expensive; however that was 7 years ago so may have improved now.
Enjoy the cruise.
Rob (Ex-cruise ship navigator)