Hi illusion lass.
As previously stated, charges once onboard include drinks, certain onboard activities (such as Bingo), onboard photos that they take, as well as certain restaurants and snacks. I'm not sure about the Navigator, but if they have a Cafe Promenade, they have certain items that are free and certain items that you have to pay for.
As far as the photos, they can range anywhere from $8 to $25, and maybe more, depending upon whether or not you want a frame, etc. You are under no obligation to purchase the photos.
In terms of the drinks, yes, as mentioned, you can either buy drinks individually or get some kind of package. The soda package is not worth it, as you have to drink at least 3 sodas per day to pay that off. I normally don't drink alcohol, so I don't know how much that would be, but again, it depends on what you order. Also, at dinner, you can get wine either by the glass or by the bottle. I prefer by the glass, as I like to try a different wine each night. That's about the extent to which I'll drink, one glass of wine every night during dinner while I'm on the cruise. A 15% gratuity is added to each drink sale, as the bartenders are those whose service is not enjoyed by everyone, unlike the cabin stewards and dining room staff.
So, in answer to your question about drink packages, you are not being charged twice in terms of those gratuities. Your prepaid gratuities include your stateroom attendant team, dining room waiter, assistant waiter, and head waiter. They DO NOT include bar staff.
I can't remember offhand what Royal Caribbean's gratuity suggestions are, in terms of amounts. If you prepaid them (either when you booked your cruise, or onboard), you will get vouchers delivered to your stateroom towards the end of the cruise. You will also get envelopes to pass out to your various team members. I usually prepay mine, and if I feel someone has gone above and beyond, I give them a little extra cash in addition to the prepaid voucher. What many people don't realize is that the crew works very hard for little money, and they rely primarily on gratuities.
An example is my Dining Room Waiter when I went on my Enchantment of the Seas cruise. By the 3rd night of a 5 night cruise, she already knew that I wanted decaf coffee with whatever dessert I chose. Having remembered that, I gave her the prepaid voucher for the cruise, as well as a little extra cash for having remembered my preferred beverage once dessert came.
2007 - Majesty of the Seas (Bahamas)
2009 - Enchantment of the Seas (Western Caribbean)
2010 - Freedom of the Seas (Western Caribbean)
2013 - Golden Princess (Alaska, roundtrip from Seattle)