What actions can a spellcaster take while concentrating to maintain a spell?
Do I understand it correct that you can only take move or move-equivalent actions? (i.e.: no attacks or other spells?) Or am I overlooking something?
You are correct. Concentrating on a spell takes a standard action on your turn, so all you can do besides that is take a move or move action. Moral of the story: cast flaming sphere first
While concentrating to maintain a spell, you can perform an amount of action equal to your whole turn minus a standard action. Which generally means a MEA plus some free actions.
The only exception is that in any case you can't cast another spell (such as a Quickened spell or a spell granted by 3.0 Haste).