Is there any reason why you want to stay in Mombasa? In town itself there is not much to do apart from visiting the Old Town Mombasa and Fort Jesus. This is a standard walking tour, altogether with the market, etc it takes approximately 2 hours if you take a guide. They take you through Fort Jesus, Old Town, Old Port, Market and back to Fort Jesus in 2-2.15 hours tops. You can also go and see handicraft factory - on the way to to airport, literally on the last stretch to the airport. Haller Park, Bombolulu, etc - these are on the north Coast, quite a way from Diani. Even Mombasa island would be quite a bit over an hour away from Diani with additional at least 40 min to the North coast if there is no traffic in town and on Nyali Bridge. Would you really want to spend the whole day traveling back and forth to see these places?
Depending on your time of flight, if it is in the afternoon or evening, you can visit Fort Jesus and Old Town on the way to the airport, just give yourself extra 3 hours allowance, leave your luggage with your transport and take a quick tour. And the rest of the time enjoy your time in Diani.
Well, good luck planning!
Tijara Beach Hotel