Since you are in transit from what sounds to be a long trip onwards from CMB, I'd make a different suggestion. Mt Lavinia Hotel is indeed nice, but it's even further from the airport than central Colombo and the Galle Face. Both are nice and I'd recommend them to anyone in Colombo for a longer stay. But what I have always done in your circumstance is go to the Jetwing Beach hotel in Negombo, go for a walk on the beach, eat a nice dinner, relax at the bar, etc. Negombo is a very short drive from the airport, so you will have more time to relax. Just my 2 cents...