There are some guidebook which cover cruise ports. Have a look round your local bookshop in the holiday guide section. You can look on the OV website for details of the 2007 excursions. It gives you the details, duration and price. I doubt if they would change much for 2008, except maybe the price.
On cruise critic under their ports of call section you will find a wealth of info on what to do in each port.
We have just come back from the OV2 med cruise.
Thoroughly enjoyed most aspects of it so hope you do too next year. We did 1 of the excursions and the rest we went on our own.
Going on your own is less hectic - no pressure for the early start and you can please yourself and obviously no way near as expensive.
The down side is that its so easy to get lost, get delayed - not see all you wanted to see.
So, a mixture is probably well worth while and maybe alternating the days.
Good luck - we thoroughly enjoyed it.
Just returned late october.our tips for the excursions would be for tunisia,dont bother to leave the ship,enjoy sometime around the pool.rome;we did the rome on your own excusion cost £34 each.this was just for the transfer to rome,about 80 mins,then did our own thing.we were supplied with map ect.had to be selective on what you really wanted to see,not enough time to see all.the tours with guides looked very rushed.this is the only tour we took mainly due to cost as we are afamily of 4.all other ports you can do your own thing.genoa ,excellent aquarium.barcelona,port bus outside ship only 3euros ret to barc centre.great metro,did the fc stadium tour ourselves saved lots money.had great holiday hope you do to.