hi TinyR,
First of all I have to say 3-4 days is not just enough to enjoy sri lanka. But as it is a Bachelorette trip you can manage that with in those days.. if you still not booked your air tickets try to book a ticket that you arrive early in your arriving day and leave the country in the evening when you departing ..so you can enjoy the most of the trip..
First day try to go Negombo. It is 20 mins taxi ride from airport and will be much cheaper compare to Colombo..and the beach is much nicer and less rough.you can have a good swim in the beach. you can find hundreds of hotel in Negombo from 1 star to 5 stars with good spas.sea food and cocktails are stunning ..make sure to book a beach hotel. (imagine you having your cocktail on the bench in beach with your sunglass on while feeling those breezy-tropical wind..: :P :P :P )
As you mention you need to do some water sports I recommend you two different places.
1) Kalpitiya- If you up to water sports in Sea this is the place ..you can have a bikini time to do Kite surfing, Snorkeling, scuba diving and Dolphin watching (you can find lots of websites if you check ) * 3 hour driving from negombo ** this can be quite cossy *** but worth)
or
2) Kithulgala
This will be totally different experience to you. this is where if you want to do 'water rafting' and this is the best place not just in Sri Lanka but also in south Asia for water rafting.. the river located inside the rain forest .the surrounding is absolute stunning and different from beach side. will be a good girly out..3-3.5 drive from Negombo
these are for second day. Same day you can come back to negambo or else you can spend you night in Colombo..both negambo and colombo is good for night out.. Colombo is more crowd (Floor by O and Clique is my personal favourites in colombo)
as you have 2 more days you can go Kandy (middle of the country) , or Galle(south, beach) . its up to u staying in same hotel or in different hotel. I would recommend you to stay totally different place from beach side in next two days..because Sri Lanka is not just beaches my be some where near to National parks ???? So you can go a good safari and see elephant gathering ,monkeys, ets..
In sri Lanka doesn't matter your are a budget traveler, back packer or a luxury seeker you can have any of them..
****But be careful with some people try to robbed you by giving wrong prices. also people who claiming they are guiders( if he/ she is a guider they have to have a licence. ) .******
****when you book hotel first check both ' https://www.agoda.com' (cheaper then booking.com) and hotel's website. because some times hotel's website giving you much better offer than those hotel booking websites..*******
*****hotel can arrange a Vehicle for your trips ..also you can find from local(can be cheaper but be careful you select a right one. I can help you on that if u want)
or anything else please contact me for any help..I would like to help you for free.... :) :) :) happy travelling