Hammer Down, There is a lot of world class road riding in the Santa Monica Mountains between Malibu and "The Valley".
It might fun to stay in Santa Monica, near the Santa Monica Pier. If you want to go upscale the hotels to look at might include Shutters, The Viceroy - if the budget is more modest consider the Wydham - if you want funky consider Ocean Lodge Hotel.And I'm sure there's a ton of AirBnB like options.
The ride routes would use Pacific Coast Highway in a north westerly direction, flat for ~10 miles through Malibu to Pepperdine University then some rollers for another 2-10 miles all along (or very near) the coast. Then you can pick a climb: Latigo has been mentioned (it's stunning), if not that then a bit farther to Encinal, or Decker, or Mulholland, or Yerba Buena, or Deer Creek. Once you're at the top of one of these you can then take Mulholland back, in an south easterly direction, towards Old Topanga. Climb Old Topanga and drop down Topanga onto PCH and roll easy on flat back to Santa Monica.
I made this map to outline a long version of the routing suggested above. I put in tiny out-n-backs off of PCH to pinpoint options for climbs along the way. You can create the mileage you want from this
http://www.gmap-pedometer.com/?r=7339151 If you wanted to do something special, shorter, still big vert, but closer consider this:
http://www.gmap-pedometer.com/?r=7339155 Couple of side notes: Riding PCH north/west is safer in the AM when traffic is a bit lighter and the sun's at your back. There are perhaps 3 short spots in the early miles where the shoulder disappears, you need to look back for safety, signal with arm, and take the lane.
I've drawn last bit of the return on the beach path - my thinking there is that it's end of your ride, you can spin easy for cool down and 12-15mph on a road or tri bike is safe within the pedestrians, runners, skateboarders, strollers, etc. etc. etc. Plus there are zero cars, the waves are breaking beside you - it's a special way to end a ride.
One more thing: This whole Malibu area was burnt to a crisp in early November. It's unique and oddly beautiful now with all the trees and brush gone, the hills are covered in a bright green short flora. As I type this it's pouring, has been for days, and 60% of the stuff I noted in my maps is closed and covered in mud and debris. My hope is that when you come we'll be back in a more typical weather patter - meaning it'll be low to mid 70s, sunny, perfect.
All the best, Ian
Ian Murray
http://www.TriathlonTrainingSeries.com I like the pursuit of mastery
Twitter - @TriCoachIan