You Don't Need To End With A Boss Fight

 There is a thought, perhaps even an unspoken rule, that an adventure of campaign has to end with a boss fight. You need to face the Big Bad Evil in some form of combat to best them and save the world. It's not an uncommon or unexpected thought. After all, most RPGs are based around combat mechanics. The root of RPGs comes from tactical combat games. The grand daddy of RPGs is D&D, a game about fantasy heroes fighting ancient gods and monsters to save the world (in the end game at least.)

You need to have a climax to the game. That climax does not need to be combat though. What else is there?

The Grand Reveal
Mystery movies and detective novels have the climax be the grand reveal. In the grand reveal all the pieces come together and the detective (the PCs) string together the events of what happened, make their accusation, lay out their proof, and the bad guy is discovered, caught, and stopped. In an RPG this could be done with collections of evidence, presenting of cases, and correct judgement of not only how to present the information but how to answer the questions that come up to try and poke holes in their argument. There's no fight in the end, just the thrill of solving the mystery and making the reveal.

Political Maneuvering
Close to the reveal, but political maneuvering can also be the climax. Rather than fighting the Big bad, the PCs can dash their plans through social manipulation. Perhaps claiming the desired position for themselves, ending a war set up to the villain's gain, or otherwise dashing the plans by accomplishing the plan before the villain, or otherwise putting obstacles in the way the villain just can't overcome in time. This is very much a social and intrigue type climax that you build up to in the end. It could work well in any intrigue setting where the end for the PCs isn't a battle to the death, but desperately trying to secure the votes of the nobility/parliament and making sure those votes are cast while protecting against the antics of the villains in return.

A Chase
A chase is not combat, but it does let all the PCs with great combat stats and physical skills be able to do things. A chase can even be a combat of sorts if you want, but by making the focus on the chase and not the fight you get to approach your climactic fight in other ways. The enemies you fight are now just hindrances to the goal of catching the fleeing villain. They don't need to be big, scary, relevant fighters able to stall the PCs. They just need to be annoying enough and able to do things that could stop the PCs method of transportation being used in the chase.