If you have a Visa to go landside then you can go to the Plaza Premium Lounge. There are 2 of these landside at International Arrivals. When you enter arrivals, turn right and at the far end is the one with a bar and massage services, but if you turn left there is the one with sleeping rooms and showers. Its comfortable and quiet but I found them a little expensive.

If you can't go landside there are the Sam's - Snooze at my Space sleeping pods. These are near gate 17 for international departures, but i've never used them so can't comment.

If you can get out of the airport there are a number of tourist attractions not to far away, without having to go into central Delhi. I recommend Qutub Minar