One of the better invocations is enduring exposure, which grants your allies the endure elements spell while also making them immune to your breath weapon. Always nice to not have to aim around them. Another good one is the magic insight, a near constant detect magic and the quickest low level identify in the game. But a fun one for a Natsu type character would be scalding dust. A gust of wind effect that ALWAYS deals fire damage regardless of save equal to the caster lvl of the DFA (usually character lvl but that can be boosted with the draconic power feat).