The cruise you're booking is fantastic ... we did it last October (2003). Great ports and the 4 days at sea total made for a real relaxing cruise (two down, and two going back to recover from the port stops).
I am emailing photos of what we did in port, as well as a gallery of ship photos. Between the Pride and Spirit, we prefer the Spirit decor. It just seems brighter than the Pride. But they are sisters, so the layout will be familiar. The Spirit's theme is different styles of interior design, which made it seem very opulent. Art and I still rave about the casino decor (Louis the 15th) because it was so fancy and so bright, as compared to how casinos normally are.
Port-wise ... Zijuatenejo/Ixtapa was our favorite. We stayed in Zijuatenejo because we did not want the Americanized resort feel of Ixtapa. My niece just got back from Acapulco cruise, and she said should would definitely do a tour there next time, rather than going on your own. It's a big city and the taxi/sales people can be aggressive. We did the cultural tour, which was very nice. Only thing was it skipped the cliff divers, whom most people rave about.
In Manzanillo, we just poked about town. Not a lot to do there, but we stopped at a beautiful old hotel there for lunch, which was very pleasant (Hotel Colonial ... it's pictured in the gallery link I am sending).
You should have fantastic weather that time of year -- high 70s and low 80s.
At any rate, here are the links I promised:
Additional links with port info (I printed out and bound as a booklet when we were on teh cruise ... made for good pool reading during the sea days):
Port and tour info:
Great Ixtapa/Z site for general info:
Manzanillo Sights to See:
And finally, yeah, I'm sure Carnival will offer this shore tour in Manzanillo:
Hope this all helps.