The best foods to make is protein swimming in lots of fat. For example, shallow-fried frikadellen. Take care to not simply use the pan as a griddle and spread a tiny layer of fat, fry in at least half a centimeter of fat, if not more. Meat and (not scrambled) eggs are good, but don't use fish.
Take care to have pure meat and not any short carbs (sugar) or long dry carbs kicking around. For example, bacon strips from bacon brined in a sweetened solution are bad. This stuff will caramelize on the pan and make it stickier than before. Bread or very doughy griddle-cake varieties (english muffins, etc.) are also bad, as they will char onto your pan.
Some doughy things are good for the seasoning, if they are not stick-prone. I find that Ruhlmann style crepes work great (2 parts egg, 2 parts milk, 1 part flour, by weight). Make sure to use enough oil before the first crepe. For later crepes, watch the pan permanently, and turn the crepe in the short time after it has self-released but before it has burnt onto the pan. You might need to play with the heat, remember that cast iron reacts slowly. Mekiza/lagos will also work well, if the pan is deep enough that you can comfortably fill it with enough oil (it doesn't have to be enough for deep frying, but you still need over a centimeter, and then enough space above it to avoid large splashes).
I wouldn't do vegetables at first, because they tend to sweat a lot of water, and may char onto the surface if you leave them alone for a short time.
Also, don't use butter at the beginning. Lard works really great for the beginning in a seasoned pan, coconut oil is also good. If you are using less saturated oils, don't do lineseed, that's very sticky when it polymerizes.
Once the seasoning "holds" better, you can expand to other things, of course. Still, your grilled cheese sandwiches are not something I would do in a seasoned pan at all, even with established seasoning.
