For that small amount, I , too, would go for the upgrade to a balcony cabin.
As for shore excursions, there are several really good independent companies out there where you might pay the same amount as the cruise lines, but it'll not be as controlled and no where near as crowded. One company we really like is Shore Trips. But depending on your ports, there are many places where doing your own thing is the best, cheapest, and easiest way to get where you want to go. We've been there and often we were able to do things and go places the shore excursions either didn't go, spent way too much time there, or spent too little time. By doing your own thing, you can control where, when, and how long.
Your travel agent should be able to help you with some options and ideas.