Just finished a Holland-America cruise to Alaska, so this is fairly current, and I think that most lines are consistent.

Beer is $4-5 each, depending on what you drink. It costs the same at the bar and in the restaurants.

Plan to pay for internet, if you need it, and for anything in the health club other than exercise machine access. There may also be port fees of about $10 each, depending on where you stop. Just about everything else was included. They give you a ship credit card rather than use cash on board, and whatever you spend goes on whatever piece of real plastic you give them. H-A added $11 per day for tips.

No indication that our bags were checked in any way, but you surrender them on the pier (or before) and they are delivered to your room. Can't say about a guitar - Seamus Kennedy certainly had one when we took his Irish music cruise a few years ago, but he was on board as a pro.