With a membersite setup a question was how members could make a room reservation, in this case a restroom reservation in blocks of hours (time slots). This search became part of our never ending research in suitable wordpress plugins.
After looking into many calendar- and agenda- plugins a very suitable plugin in this simple non-commercial wordpress member area is wp booking calendar. The free version made use see all the options available and integrate this in multiple ways. Together with the wp-members plugins this calendar is a good option to let members book their space or appointments.
With wp-booking-calendar several other calendars are still on top of our list, but for more commercial type sites, like the events calendar
and the wp-booking-system
For less commercial and simple management the simply-schedule-appointments plugin is also recommended, a compact functionality and quickly to setup. The complete look-n-feel might make it a bit standalone in your theme design.
So far a nice selection of calendar and agenda plugins, more plugin research at the github project page