The ideal domain to park is one that recieves a lot of traffic and has a high cost per click.
Something that you could develop would be domains like:
ProductDomains - e.g. USBDrives.com - you can use affiliates
DirectoryDomains -e.g. TorontoDentists.com - dentists pay for listings.
As with a real "site" you are going to have to do SEO, or some marketing to get more visitors if the type in isn't good.
Skinny