Hey Anders,
This thread has a lot of info about creating a calendar using a repeating group. Based on this thread there was even a plugin created (go to page test_plugin to see an example and ignore the color 
).
Maybe it is an option to color the day based on the timeslots available and allow users to click on it. When they click on it you can show a popup with available timeslots for that day together with the option to book a certain timeslot.