Port Arrive Time and Boarding number are done when you do your online check in.
While all 3 rooms won't have to check in together, I believe all persons in any one room must be present to check in.
"Must do" really depends on what you want. As to what extra costs there are here's a list:
1. Gratuities (added to your onboard account automatically)
2. Parking/transportation
3. Porters
4. Room upgrades
6 Cellular phone calls (will be billed later to your phone bill)
7. Ship to shore calls from your room phone
8. Welcome aboard packages (flowers, candy, etc waiting in your cabin)
9. Duty Free Alcohol and cigarettes purchased during cruise
10. Alcoholic drinks at the bars and from roving servers poolside and in theatre
11. Tastings and Mixology
12. Wine by the glass/wine packages
13. Additional tip (over the auto gratuity added to all drinks) for bar servers
14. Tips for room service
15. Soft drinks and candy/snacks from room service/theatre snack bar
16. Specialty coffees
17. Bottled water (sparkling or still)
18. Candy and ice cream from Vanellope’s on the Dream
19. Smoothies from the bar in Senses Spa on the Dream
20. Upcharge food items at the adult clubs
21. Adult dining (Palo, Remy)
22. Shore excursions (including cabanas and rentals at CC)
23. Tip for Castaway Cay cabana host (cash)
24. Stamps from Post Office at Castaway Cay
25. Excursion tips
26. Spa treatments (plus 18% gratuity)
27. Bingo
28. Sports simulator
29. Photos (individual and packages)
30. Internet
31. Art
32. Purchases from the stores: clothing, sundries, jewelry, souvenirs, cameras, etc.
33. Pins sold at trading pin sessions
34. Cigars (Meridian bar)
35. Hair Braiding
36. Laundry room and send-out laundry and dry cleaning
37. Onboard booking for future cruises
38. Disney Vacation Club membership
39. Adventures by Disney
40. Medical care
41. Nursery (for under 3’s)
42. Kid’s club magic bands (if you want to keep them, you lose the deposit)
43. Meeting room rental
44. Onboard or Castaway Cay wedding/vow renewal (currently $3000/$4000)
45. Tuxedo rentals
46. Duties at customs at end of cruise if you've made a LOT of purchases abroad
47. Fuel supplement charge if the price of oil is above a certain level (never happened yet, that I know of)
48. Fish Extender program gifts (if you participate)