Is there a way to end the war other than joining the Stormcloaks or Imperials?

No.

There is no way you can complete the war part of the game without taking a side. There is no such option in-game. The war part between the factions must be completed taking one of the sides and doing the missions.

No, at least not in an unmodded game.

The civil war plotlines have no "third option". You either side with the Stormcloaks and save Skyrim, side with the Imperials to control it for the elves, or stay out of it altogether.

If you're playing on PC, there are likely to be mods that allow this.