Hooks and Cliffhangers
One thing that you should have plenty of is hooks, rather than spend all of them during the session, you can save a couple for when things start to drag on.
While your party is doing one final survey of the dungeon, they find evidence of a prior group's investigation and a journal.
That journal can be core-plot relevant or side-plot relevant, but whatever it is, it can be used to distract the players from the slog. It is something new and interesting that they will question immediately (hopefully) and likely steer them towards things both you and they find interesting.
When the session is about to end, you can use this technique as an opportunity to place that interest onto the next session.
In the journal, you find that this past troupe was sent by King Ronald to hunt the dreaded Grimace. It's not anything like what you've found during your time within the dungeon. However, as you read on, there's suddenly a deep and echoing laugh emanating through the dungeon around you. And that's where we'll leave off.
And that's where we'll leave off.
There is an art to this, deciding when in the middle of a hook to stop the storytelling. You want to give the players just enough info for them to know something is happening next, but get in the way of them being able to resolve it during this session by fading to black.
Their minds are already thinking "what would be my next course of action" but without the opportunity to get the feedback of actually doing anything, this will build up anticipation for the next session when they will actually get to see the plan that's flashed through their heads, or the courses of action they've talked about amongst themselves between sessions, or the crazy ideas they've thought of in the meantime that might actually be worth trying.
Don't do this too often.
Sometimes, it's important for sessions to just have lulls in them. Once you've overcome a major arc, the players don't need to be put to the blade again so soon. Repeated pressure to "do the next thing" can lead to player burnout and might be more stressful. This is a balance you need to learn with your players and it's not something I can really give a good rule of thumb for. Keep an eye on your players and see if they're trying to accomplish something in game that is getting interrupted by these hooks. If players are willing to pick their own pace and trajectory, this saves you some work and guarantees they're invested.