Design To Handle Multiple Similar Model Class
I would suggest the following potential improvements.

Change the Quotation class to the following

public class Quotation
{
    private Customer customer;

    private List&lt;BaseEvent&gt; events = new List&lt;BaseEvent&gt;();


    // Getters and setters to access the two entities above.
}


So now you have divided you elements into specific entities like Quotation, Customer and Events.

Adding more details to a Customer or adding a new Event type, don't require change to the Quotation class, but only changes to the specific entity.

On the side, you might also find it beneficial to look into a simple visitor pattern for the events and provide algorithms to etc. calculate total price, date collisions, number of participants etc. etc.

Enjoy.