Do you want everyone to be able to edit the bookings, or just view to see if the rooms are free or not, and therefore find a suitable time for their meeting? If so, you COULD use Excel and just keep the file somewhere public. Anyone opening it can view a read only version, you can password protect it etc.
However, GOogle calendar is by FAR the easier way. Set it up to VIEW ONLY and then people send you emails/phone calls to book in a meeting. Not very automatic, but it will do the job.
If you’re looking for a system where everyone own their own bookings and can’t edit other people’s then you could still use Google Calendar, but youo’d need to setup a calendar for each room, and each user would also need a calendar. When someone wants to book the room, they input a new appointment in THEIR calendar and invite the room to that booking so it shows in the room’s calendar. The admin (you?) can then accept the invite to confirm the booking (but even if you don’t accept it, it should still show in the booking so other people don’t book over it).
IT’s complicated, but it can be done. You can also use something like calendly (there’s a few out there - google ‘online scheduling free’ and see what comes up!), which allows people to book times through a link/website and it will populate an Outlook or google calendar. Each room would need a google account for this I think but it will prevent double bookings, which the Google calendar option may not.
Happy to work with you to find exactly what you need if you want. Does this sound like it’ll do what you want/need? If not, reply and we’ll see if we can work out something specific that fits the need!