I'm staying in Mombasa for 6 nights at distant relatives from 27th to 2nd jan. Is this long enough????
Depends on how you decide to spend the duration.
There's lots to do at DR eg Yoga, Village visits, Musafir Dhow Cruise...etc.
DR is famous for New Year's Eve Bash.
Malindi Town is worth visiting for shopping, sight-seeing, bars/restaurants, Crocodile Farm..etc.
MBA Old Town & Fort Jesus for a day trip is worth while.
The main side attractions on the Kenya coast, aside from the beaches, are the national parks and reserves. You don't mention an interest in going there, so I'd say that unless you're a serious sun worshipper or seriously into diving or kite surfing, you'll find it long enough. The Kenya coast isn't packed with side attractions of the kinds you'd find in some other resort areas around the world, such as world class dining districts, malls, water parks, etc. (And I stress world class.) It's a very quiet place, and if anything quieter than 20 years ago when tourism was booming.
You will absolutely love it there. The two guys that own it are Australian and French and met at Uni Queensland. They have the best NYE party that runs for 2 solid days and nights, and have so much happening at that time of the year. You'll leave with an epic hangover.