When you present your players with a clear course of action at the start of a session — the adventure, in other words — and they say, “Nope, we’re not doing that — we want to do this instead,” you have an interesting decision to make.
Do you improvise an entire session, or call it a night?
First, decide if you can wing the session or not. This is a crucial decision point, and one that’s based on a variety of factors: your comfort level and experience with improvisation, whether skipping what you had planned will really screw up the game and your group’s social contract, to name three big factors.
If you can wing it, wing it. For whatever reason, your players are more interested in what they came up with than what you came up with. Don’t take that personally, just file it away for consideration after the session — right now, start improvising.
For better or worse, every GM should improvise an entire session at least once. It’s a major GMing milestone, and it’ll teach you all sorts of thinks about the craft of game mastering. It might be nerve-wracking the first time, but it could also turn out to be a lot of fun.
If you can’t wing it, don’t. There’s a line between getting out of your GMing comfort zone and biting off more than you can chew (and possibly running an un-fun session in the process), and you’re the only one who can decide where that line is for you.
If you decide not to improvise the session, you have two options: stop for the night, or explain the situation to your players (and possibly run the adventure you prepped).
In the first case, you’d say something like, “Awesome — that sounds like a lot of fun, but I’m not prepared to run that tonight. Let’s play board games instead, and I’ll prep that for next week.”
In the second case, it might go like this, “That sounds like fun, but it isn’t what I prepped for tonight. I don’t think I can wing a whole session around that, so if we don’t play what I have prepared, we’ll have to do something else tonight. How should we handle this?”
Personally, I recommend the second option — laying your cards on the table and including your players in the decision process is generally the best choice.
What do you do in this situation?