How to manage reservations with Drupal for something that is not something like a room?
For Drupal 6 there is a stable release of Simple Reservation module.  


  This module provides a simple and easy way to reserve items which can be created by the administrator. Examples for it's usage can be hotel rooms, boats, cars, airplanes, basically anything you can think of which can be reserved by one individual.


For Drupal 7 you can try Reservations API.


  The core focus in on the Reservations API that enables any Content Type to be reservable based on a variety of default settings. A Reservation -> Confirmation -> Checkout -> Checkin workflows is included, but it is created entirely using Views. Additional modules can enforce custom validation, add pricing, or add additional availability to the basic hours of operation.