You're following up this one day visit with a cruise, right? Well, there should be ample opportunity for character meet and greets on the cruise, so I'd focus on rides. I'd concentrate on those that have what are traditionally the highest wait times. Those would be:
7 Dwarves Mine Train
Peter Pan's Flight
Enchanted Tales with Belle
Space Mountain
Splash Mountain
Big Thunder Mountain
Now, I just gave you 6 rides that are on my "must Fastpass" list, and you only get to reserve 3. You might get one more FP after you use up the scheduled ones, but no guarantees. You kind of need to base your selections on your and your family's preferences. For example, classic rides (Peter Pan, Space Mountain and Big Thunder) or newer rides (7 Dwarves, Enchanted Tales and ...well...Splash isn't really new...)? Also, a toddler may not be able to ride all of these (or want to).
Mesaboy on this Board has an excellent Fastpass tutorial, but I have no idea where his thread is to link you to just now.