Info from those who have been on a cruise.

I've been on 4 cruises (certainly not a veteran).

Biggest "hidden" cost is booze. It's ridiculously expensive, imo.

I've sailed on Princess (2), Norwegian (1), and Royal Caribbean (1). None of them allow you to bring alcohol on board. But...what we've done on all of our cruises is buy some the night before (we always arrive in our port city at least one day in advance) and buy whatever booze we want. Then we buy water bottles and fill them with vodka or white rum and buy ginger ale and fill those bottles with dark rum. We put these in plastic bags in our luggage and have never had an issue. Every(?) cruise we've been on offers an unlimited Pepsi/Coke pass for like $50 for the week. Sure you have to go back to your room for every refill but it beats $18 for a "double" rum and coke.

It depends on the cruise line (and duration?) but I think you are allowed a certain number of bags but I can't remember. My wife and I have always packed one big suitcase each along with a carry-on type sized bag.

It depends on where you are going. Everything on board can be charged right to your account which is very convenient but it can add up in a hurry. We always bring cash to tip our head waiter (if we do assigned-time dining) and cabin steward. Almost everything done at ports of call can be done with credit (it's 2013....) but there might be some local mom and pop places that don't take plastic - again, it depends where you are going, what you want to buy, etc.

Alaska is absolutely magnificent. I would highly recommend either a balcony or a mini-suite room for Alaska, especially if you are going to stay on the ship (as opposed to doing an excursion) during the scenic cruising through the glaciers. We've only done balcony rooms and it's worth it, imo. On our last cruise we were upgraded to a mini-suite for free. The extra room was really nice and my wife said she's not sure if she would go back to a balcony again.

The excursions in Alaska are relatively pricey compared to Caribbean excursions but they are well worth it. I went on a 4 hour salmon fishing trip while wifey did a high ropes course, we did a glacier excursion where we got on a smaller boat and zoomed in front of the cruse ship and got much closer to the glaciers than the cruise ship, and we did the White Pass railway tour. All of them exceeded our expectations and are memories that neither of us will ever forget.

I've sailed Royal Caribbean and Carnival over the years.
Planning a Hawaii or Western Caribbean cruise for next year.

There are plans on some cruises to have the tips already accounted for.
There are also the soda and alcohol plans available. Sometimes the drink plans drop in price the second day of the cruise.
There are plenty of free things on board but there are MANY upgrades that will cost. Choose wisely.

Every cruise I have been on so far there are drinks in the room. CHECK THE PRICE SHEET WITH THESE DRINKS!
They are typically charged to your room based on what is used and are expensive. Some people don't look, just grab and then get charged for it. We have always had the drink cards which was unlimited refills at the many drink stations around the ship.
My teens used them frequently.

Room service food can be had most anytime. You will have to check your individual ship for what is free, what is charged and times of service.
Ice cream and pizza could be found 24 hours a day in one certain "cafeteria" type area.

Typically there is a gym and it will cost to get into it.
I used a free track to run and met several other people running and doing body weight exercises. Normally around sunrise.
Bring your camera if you run on deck at sunrise. It is a spectacular sight and a rather inspiring run.

There are typically one or two "formal nights" in a designated dining room. The cruise would "threaten" to not serve under dressed patrons. The reality is that under dressed people still showed up and were served. It's fun to dress nice for an evening or two.

Shore excursions can be pricey. It's also a kind of cattle call. Transportation and times are normally handled very well since these places are in with the cruise companies.
You can also do your research and find your own excursions to do. Possibly even cheaper. You have to be mindful of your times and transportation to get back on ship before it departs port.
I've done excursions with Mayan pyramids, swimming with dolphins, submarine rides, snorkeling and snuba.
We have also done on our own on-shore exploring, shopping, snorkeling, drinking and eating.

Check with cruise message boards and tripadvisor.com for more detailed info.
Google is your friend.

There is no mandatory excursion .. I mean you can stay on ship and relax. I hope. Gosh I can see where these trips add up fast!

Correct. This is what my grandparents used to do after they'd done dozens of cruises. They had seen everything they wanted to see at all the ports in the Caribbean so they just stayed on the ship when it was at port. The pools areas, dining areas, and lounges are (usually) MUCH less crowded since lots of people decide to get off the ship to do whatever.