You do it during a clerkship, not between. Have an abundance mentality. Have confidence.
Many students try to be sneaky and plan 'travel days' where they can go see another program under the radar, but that's cowardly and easy to read through anyways. The good programs will generally choose a well-qualified student who knows their stuff and confidently tells them that s/he will be gone a couple days ...as opposed to a barely-qualified wimp who tries to kiss the bottom of the director every single day of the extern month.
I would - and I did - just tell (don't ask!) a program at or before the start of your month that you are taking a Th + Fri or whatever to see another area program. I did this on 2/5 of my externship months, maybe 3/5. You have to find out what day(s) the director of the program you'll visit is in clinic and set it up. There is no need to rub it in the faces of the clerkship program attendings or residents, but you don't have to hide it either. Don't feel bad if they're giving you free room/board or food; your clerkships are YOUR clerkships.
Basically, if they are confident they have a good program, they won't care... they might even tell you which other programs in the city are worth a look. I matched at my top choice program despite doing this (visiting two others in Detroit during my clerk month), and they encouraged visiting the other top programs in the metro. Likewise, if you are confident you're one of their best students, you shouldn't feel bad about it. 90% of the days from an excellent student beats 100% from a mediocre... every time. Do you really wants a program that picks students based on their sucking up and perfect attendance? No way. Good programs know you have limited clerkships, hundreds of residency hospitals, and they know you already picked them as one of the 3-6 or whatever pre-interviews clerkships that each school allows. 👍