As far as the soda cards (for kids), they are cheaper to purchase the day you arrive on the ship (are $14.95, I believe). You can buy them at the Information desk. Whereas if you wait until the second day, it is $19.95. I just returned from the Carnival Triumph (yesterday).
jeff as far as the drink coupons you need to buy them before you sail they cost 19.50 for 4 coupons. we sail in oct. and they saved us some cash .it cost 3.75+15% tip for a good mixed drink and 4.75+15 % tip for a frozen drink it is alot cheaper to use coupons.plus you do not have to take out you sail&sign card or sign for drinks you just give them your coupon.
You can buy the drink coupons on carnival.com if you know your cabin and booking number. You can probably order them on the 1-800 phone number too. The soda cards are purchased on board the ship.