The tours that say "INCLUDED" are no additional cost to you but still require reservations so that Viking knows how many buses and guides to line up. The tours that don't say "INCLUDED" are optional, paid tours. Generally, they require payment because Viking is buying your admission ticket, arranging for transportation, paying for food (depending on the excursion), etc. The cost usually ranges from about $50-$150 (USD), depending on the activity and whether any meals are included, but they can cost much more than that if it's a specialized activity like a helicopter tour.
You can look up Viator, Get Your Guide, and other private tour operators who offer the same or similar paid excursions, and they will often be cheaper even after factoring in your own transportation costs. But unless you find something significantly cheaper, I'd stick with Viking just to avoid the risk of getting left behind if something goes wrong and your return to the ship is delayed.
