Need more than just a boss fight for my finale

So, I'm starting the planning process for the finale of my campaign. The boss fight is down. I think it's very nice. However, a boss fight isn't enough for an entire adventure. The boss doesn't have any minions, is currently hiding, and burning some souls. I don't run a serious table, so there aren't any character arcs to tie up, and a dungeon crawl wouldn't fit super well.

Any advice is welcome 

Non serious table?  Pick a hacknied movie trope and fill out the end scene:

Freaky Friday:  Players are randomly body switched and have to play each other's characters.

Alien: Kill the bad guy and have some kind of chest burster mini alien pop out and they have to chase it down and kill it.

Horror: Bad guy just won't stay down.  Just when you turn around the body gets up and walks away.

 Or some other favorite you have.

So, what sort of setting, where is the boss, what sort of boss is it?

No minions is fine, but maybe they have guard animals, or some traps which the party could get wound up in?

What level is the party? No point suggesting a deep pit to somehow get across if they're level 15!

Quote from ThorukDuckSlayer >>

So, what sort of setting, where is the boss, what sort of boss is it?

No minions is fine, but maybe they have guard animals, or some traps which the party could get wound up in?

What level is the party? No point suggesting a deep pit to somehow get across if they're level 15!

 It's forgotten realms with some tweaks. I call it the remembered realms. The boss is essentially a spellcasting Terminator (a flame skull controlling a magic item-filled robot body) who's trying to burn some souls for plot. The world doesn't like that, so it creates elementals to destroy his machine. He hides from them by hanging out around the storm king's Castle, and hiding the magical signature of his machine among the constant magic storm.

Maybe. Perhaps deactivating the machine before the fight, or convincing the storm king to stop his storm so the elementals can find him. The boss does currently assume them dead, after trapping them in a living mountain 

They're level 5

If he is making use of the Storm King's defences, like a parasitic fortress, then it would make sense for the party to be beset by lightning and thunder-themed enemies on their way there, which exist to protect the Storm King's fortress. There's also the environment, which I imagine to be quite hostile.

The fight sounds like a good option for clashing objectives, rather than mortal kombat. The BBEG is trying to activate their machine, whilst the party is trying to kill him.

I know you said you don’t have character arcs to tie up, but you could still run a little epilogue. Assuming the party wins, you could describe the effect on the world, maybe let the characters go back to town and have a celebration for them. 

And at the end of a campaign, I always like to go around the table and let the players say what their character does next. It lets them have a nice send off, and can help seed the locations of them as NPCs for the next campaign.  

Add a maze of flames and machinery and whatever that randomizes itself like the labyrinth, and is very punishing to navigate. When they defeat the boss, it will feel earned, not by the difficulty of the fight, but the trouble of getting there.