Big ideas like story arcs and how they fit into the bigger overall plot. I really enjoy planning how those all weave together as I'm doing a long run fantasy with an ensemble cast so there are a lot of moving parts that need to fit together.
In the planning stage, mapping out how smaller stories fit within the larger one is thoroughly enjoyable. But when I get down to the nitty gritty of new character designs, I throw a minor internal tantrum like a toddler who is too excited about life to take that nap he needs.