Plan fun sessions, plot is secondary
This is somewhat of a challenge to your premise, but bear with me. adonies' answer covers a lot of particulars that you can include. I will try to take a slightly different perspective.
A session that doesn't advance the plot doesn't need to be planned any differently to one that does. When planning your sessions you should be trying to make it as fun as possible, if that happens to also advance the plot that's a bonus.
For some groups, fun is derived solely from completing goals and progressing the plot. If that is the case with your group, you may want to reassess why you are having a low-plot session at all. As DM you can control the passage of time and you can quickly progress to a point where the plot will begin moving forward once more. Thus avoiding this situation entirely.
Other groups, instead take enjoyment from the entertaining or dramatic moments within sessions. "Remember that time when..." type moments. In my experience these moments are impossible to plan for. Sure, beating a BBEG is memorable, but the ridiculous thing the rogue did was the real highlight of the session. For these kind of groups you want to present them with many and varied situations to give them the best chance for something exciting to happen.
Most likely you have a group that is somewhere in between these two, or you will have a few playing in each category. This is why campaigns tend to have an ebb and flow to the tension of sessions. Some are high stakes with plenty of drama, some are silly side quests or shopping sessions filled entertaining NPCs. Whatever happens in your session your goal is to make it fun for your players.
This session doesn't sound like it is lacking plot
Let me just highlight one thing you mentioned:
their characters are probably in trouble for causing several messes in town
To me, this is a hook enough for an entire story arc, let alone a single session. In fact, I have done it. My players got into a scuffle with some locals and decided they needed to leave town in a hurry. I turned this into a 3 session arc where they had to fight tooth and nail to regain their freedom after doing a dodgy deal with a slaver to smuggle them out of town. Sure, it wasn't advancing the plot as I had planned, but the players enjoyed it.