For our meeting rooms, and external community users of our school facilities, we use MIDAS, as it allows staff/external hirers to check room availability and submit booking requests online, which our facilities manager can then easily approve/reject with a couple of clicks (the system then automatically sends an email notification to the original requester too)
I believe MIDAS can also be configured to allow for booking pre-define "time slots", or durations (i.e. 15 minutes), although we don't use that particular facility for our needs.