Inventions is a rather nice buffet, and I get the idea that all the seafood is there because it's generally more expensive food to help justify the higher price. There are usually plenty of characters around (but only 1 at a time), last time I ate there I met 4 characters in about an hour's time, so they rotate them around fairly quickly.
The fish is quite varied (shrimp, salmon, etc, but also hot fish dishes), but there is also normal meat (slow cooked lamb, roast beef, pork, chicken wings, all sorts really), there's a number of hot vegetable dishes as well (also mashed potato and such).
And there are mickey shaped chicken nuggets and pasta for the little ones too for instance.
There's generally a variety of different salads available and a wide variety of dessert items (little jelly puddings for instance, and mousse, different types of cake).
As for upgrading to a higher plan, Premium, which would cover inventions completely is a fair bit more expensive than Standard. You'll need to either call or mail Disney to see what the actual cost to upgrade would be, but I'll list the plan prices below.
For Standard Half-Board you would have paid £22 per Adult, and £11 per Child
For Standard Full-Board you would have paid £39 per Adult, and £19 per Child
For Premium Half-Board you would have paid £48 per Adult, and £24 per Child
For Premium Full-Board you would have paid £83 per Adult, and £41 per Child
