Congrats for choosing to visit Goa. 18 days is a lot of time and I'm guessing that you'll have a fantastic vacation. The thing about Goa is that there's never a wrong time to visit unless you're visiting only for the seafood or/and sunbathing in which case you must avoid the monsoon.
You can definitely enjoy water sports in Goa. Be prepared to negotiate the price a lot. Also, make sure that you're in safe hands while undertaking these activities. You may choose from jet skiing, float boat rides, banana boats, parasailing, catamaran sailing, and bumper boats. Calangute, Baga, Candolim, Colva and Bogmallo beaches offer water sports. You may also go for kayaking (Nerul and Siolim), wind surfing (Morjim and Dona Paula), and rafting (Dandeli).
The other fun activities which you must indulge in are:
1. Eating at continental restaurants: Try Thalasa in Vagator and La Plage in Morjim.
2. Visit the Saturday night market at Arpora.
3. Boat ride on the Chapora river: You may get a chance to see dolphins.
4. Visit a spa and massage parlour: Make sure that you check for authenticity. Snip is a well-known massage parlour and is located at Calangute.
5. Experience rural Goa: Leave behind the hustle and bustle of the city and visit the small traditional homes in rural Goa. You're in for a treat of traditional food and the true flavour of Goa.
6. Caves: Though they're not well known, Goa caves are worth a visit. You may pick from Arvalem, Lamgau and Rivona caves.
7. Cruise: Take a cruise down the Mondovi river. Pick the evening time and you're in for a visual treat thanks to the sunset.
8. Visit the butterfly conservatory in Ponda.
9. Dudhsagar falls: Check the local train timings and plan your trip accordingly. It is advisable to dedicate a day for this trip.
Mix these with your lazy days relaxing on the beaches of Goa and you'll have an unforgettable Goa trip. You may choose to hire a car for your visit. Make sure that you carry extra copies of your identification papers since you'll have to deposit your original ones with the car-rental agency.