Also, I think it would depend on how long and how often these learning sessions will be. For example if it’s only a half hour once a week, you wouldn’t want to do something where in the allotted time you barely get anywhere and by the time the next session comes around you have to review what you did the previous session, and you never really get anywhere. In such a situation you would want something that is self-contained where you can do one piece each time like the Pele Yoetz (which you could actually probably do five pieces each time, but I think that the Pele Yoetz is not at all what you’re looking for). But if the sessions are every day or they are significantly long then you wouldn’t really have any limitations in this particular area.
Do you want something more philosophical, such as Emunah V’deios, Moreh Nevuchim, Chovos Halevavos, etc.? Do you want something from the Rishonim, or from a more recent thinker? Do you want Mussar? Hashkafa? Machshava? Halacha? Lomdus?