The way I (and a fair few others it seems) do business is by selling multi day events as a package of single events as well as the single events.
Unfortunately none of the workarounds available are good enough.
What I'd really like is to be able to set up an event as a group event and be able to link this event to other events so that if someone registers for the event a script runs to automatically register for all the linked events as well
Practically I don't mind whether it is a grouped event that selects sub events or whether a sub event has the option to link to a group event. .