Well the whole of the south of England, from Kent to Cornwall, has something of interest.
If I were them I would look at the map and see the easiest place to get to as they are only away for 3 or 4 days.
You dont want to spend the whole day driving to the tip of Cornwall and a few days later have to do the long drive home again.
It also depends what sort of thing they like, how old they are, and if they want to be in a city or town, or in the country.
Brighton is a young exciting city with plenty of nightlife if they want that. Bournmouth, Portsmouth and Newquay are lively seaside towns.
Devon and Cornwall has beautiful coast and beaches, with some lovely towns and villages (Padstow, Falmouth, Torquay, St Ives)
The Isle of Wight is rather romantic and is small enough to drive round in a few days.
There are LOADS of tourist web sites about visting southern England. Just search in Google for various things like "Visit Cornwall" and all sorts of pages come up.
Here are some examples:
http://www.cornwalltouristboard.co.uk/
http://www.englishriviera.co.uk/home/index.asp
http://www.islandbreaks.co.uk/
I would say a trip to the Isle of Wight would be something different and a nice way to spend a honeymoon. Go visit Osborne House, the seaside home of Queen Victoria.
http://www.english-heritage.org.uk/server/show /ConProperty.205