Hi, Emily - besides the great info from Seth, Jonathon & David - I would add picking dances that you yourself are very comfortable with, and look at the transitions.
(A good starting place is looking at a 16 count neighbor interaction for the A1 for a less experienced group to have time to adjust. Not much to adjust from do-si-do & swing to balance & swing...)

Call a little early.
Give directional hints that you might not have needed if you had done a walkthrough (ex. - "look across" before the next action if appropriate, or "end facing down the hall" after a swing that precedes "down the hall four in line")
Say something like "listen up" as you transition into the new dance.

They are not that varied, but I have often used Simplicity Swing (Becky Hill) and The Nice Combination (Gene Huber) as a 2 dance no-walk-through medley toward the end of the evening. It is nice to end with a down the hall.

Airpants is another nice simple dance.

For a first time, two dances might be enough to have fun with; go with what you feel.
As suggested, think about what figures or combinations of figures have already been danced that night and will seem familiar.

If transitioning to a dance that requires having to find a new same role dancer across the set to start (maybe not recommended when trying it for the first time with less experienced dancers...) think about how to cue that
ex: "NEW Robins allemand"

Have fun and best of luck,

Valerie

Hi David, Jonathan, and Seth,

Thanks so much for jumping in on this question about medleys and giving me
some ideas to chew on! I've danced some medleys but not since before the
pandemic and as mentioned, have never tried calling one. I definitely want
it to be a success.... no flops! Great point about having all the
choreography be familiar and the dances not be a challenge. That was on
my radar. Hadn't thought about putting a dance from earlier in the
evening in - cool idea Seth!

Thank you :)
Emily