An 8-hr layover means you can't really go out and see the city. Delhi's T3 has lots of hang-out options. Though shopping would be a much expensive affair there, you can pass some time in the food court and spa.
An amazing sleep-on facility has been introduced at T3, where you can rent pay-per-use lounges and sleeping pods! http://www.newdelhiairport.in/sleeping-pods-airport.aspx
There is also a hotel located within T3. Here's the link: http://www.newdelhiairport.in/eaton-smart.aspx almost 5 years ago