We recently installed BookThatApp on a site offering equipment rentals. In this case the store owner wanted to make the process for booking a product to be as simple as possible for the customer.
We set up the booking form with a start date and an input for the number of days they wanted to rent the product for. Since the rate for the product is the same every day, we used the standard Shopify quantity field for the number of days.
One final wrinkle we had to account for was making sure you couldn't book the product if it wasn't available. BookThatApp already blacks out unavailable days in the datepicker, but in this case you can change the duration with the quantity. To implement this we used the BTA jquery library to check availability when calculating the finish date. If it isn't available the submit button is disabled.
If you'd like to implement something like this please feel free to browse the code. It also available here: https://gist.github.com/gterrill/5375390.