Three optional elements to an event

Can someone please help. If I have an event which consists of three optional elements (say Session 1, Session 2 and Session 3), what is the best way for me to display this? Ideally I would want the three options to show in a checklist after the user clicks Register Now for the event, allowing them to choose the options they wish to attend and then checking out. I want to use custom fields for this, rather than treating them as three separate events, however I have tried to follow the instructions with fee fields but all I am getting is the word "Array" showing in the event properties, and when clicking Register there are no options showing. Any advice? Thanks

Setting up a checkbox custom field as follows should work