I haven't been to Alaska, but have cruised quite a bit. I've booked excursion both through the ship and on my own. Keep in mind that if you book on your own and you don't get back to the port on time the ship will leave without you and you are responsible for getting yourself to the next port of call. If there is an excursion that I want to take that only lasts a few hours and is fairly close to the pier I tend to book on my own. If its a long excursion (ie we're in port for 8 hours and its a 6-7 hour excursion) I book through the ship. I also book through the ship if I have to travel inland for more than an hour.
If you decide to book on your own make sure you book through a reputable company. Check the BBB in that area and see if there are any complaints and try to get recommendations from other cruisers. Check to see what their policy is if they don't get you back on time and make sure you get it in writing.
Anything I'm booking through the ship I book on line ahead of time. Popular excursions tend to book up quickly.
Soda is not free. You pay for it, even with your meal. If you drink a lot of soda I would get the soda card. If you're like me and only will have one or two then buy it by the can/glass. I usually buy the soda cards just for the kids.