Sending clients a calendar item

How can I easily send my clients a calendar item, either by email, or allow them to download it? I have an event coming up. I have a date booked. I send out invitations to my contacts. When they click on the link to book themselves on, I want to give them the ability to download to their calendar the event details. I assume I need to provide it in 3 formats : outlook, google and ical (these seem to be the choices I always see). What is the easiest way to do this?