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