lewalters,
A few thoughts on the options you mentioned.....
- Portofino & Santa Margherita - both are scenic towns but they're not exactly close to La Spezia so the travel time there and back will be longer. In addition there's not really a lot to see in Portofino except for the pretty harbour and some posh shops which most people can't afford anyway.
- Florence & Pisa - Pisa is not worth the effort for such a short stop (IMO) and it will detract from Florence which is a more worthwhile destination.
- Florence all day - if you're interested in Renaissance art and prefer large cities, Florence has a lot to offer including the Uffizi and Accademia. However, again it's some distance from La Spezia so you'll be spending more time sitting on a bus. Florence is the capital of Tuscany and a very scenic location on the Arno River.
- Cinque Terre - Of all the destinations you listed, this will be the easiest of all the options as it's so close. You can easily tour the five towns on your own, and tailor the time frame to what you feel comfortable with. All you'll need to do is take a Taxi or whatever from the dock to La Spezia Centrale, where you'll connect with the local trains (I don't cruise so don't know what the best method is). The rail trips between towns are all short. For example, from La Spezia to Riomaggiore (the closest of the five) is only about 8-10 minutes.
If you choose Option 4, I'd recommend doing some research and pre-planning so you get the most out of your time there. As someone else mentioned, the local trains may be hot and crowded at that time of year (often the case when cruise ships are docked in the area). As the trips are short, that's usually tolerable. The towns can also be somewhat crowded on cruise ship days (especially Vernazza), although Monterosso (north end) is larger so seems to tolerate the crowds better. You could perhaps take the train directly to Monterosso and return to Riomaggiore on the boat, stopping at each of the towns (except Corniglia) for a short walkabout. At the end of your visit, take the short train ride from Riomaggiore to La Spezia Centrale, and then return to the ship. I can suggest restaurants in Monterosso, but don't have much information on the other towns.
For any of the options, it's also a good idea to wear a Money Belt under clothing (don't access where others can see), as where ever there are crowds of tourists, the pickpockets will be circling like vultures.