I have just spent 18 months working in Sri Lanka at Colombo's domestic airport on the other side of the city but flew in and out of the country many times during that period. Sri Lanka is a reasonably good airline with comfortable economy seating. Some of their aircraft have a small section at the front of economy with about 8 rows of seats. It is separated from the main economy cabin and gives you a bit more relief from the usual economy noise etc. Well worth asking if this is the case on your aircraft. The intl airport has been extensively rebuilt and extended and is now quite pleasant and clean on the airside but like all airports its not the best place to spend 8 hrs. I suggest you clear immigration and take a taxi to the Mount Lavinia Hotel where you can relax in one of the best hotels anywhere. It is about 1 hour drive from the airport but you would pass around the outskirts of the city so would get a look at the local scene on the way. Fare would be approx USD20 each way so don't let them charge you more than that. The food and drink is very cheap and very good and you will be able to use the pool and their private beach for about USD5.00 for as long as you like. I would suggest you take a small bag with you for a change of clothes and swimming gear if that's what you like. If not, it is very pleasant to just sit and enjoy the surroundings. Check if there is a full moon on the day you travel!! If so then it is a bank (and religious) holiday in Sri Lanka and no alchohol can be sold by law anywhere except for small house bars in the big hotels and that is only for full guests.
Best of luck.