Check out Long beach park for waves, restaurants, and their boardwalk.
For the shopaholics out there, visit Tanger outlets in Riverhead. It is huge with many major designer and brand stores at discount prices. Visit the link to see their offerings. It is a bit of a drive (one hour), but for those who love to shop for designer goods, it will be worth it.
Oheka Castle is a historic mansion located on the famed Gold Coast of Long Island between New York City and The Hamptons. Oheka Castle has beautiful grounds and a restaurant with beautiful views. Dine and/or tour the grounds by making your reservation ahead of time.
Huntington Village is a great downtown area to spend a few hours. Park your car and explore it on foot. Grab a hot slice at either Rosa'a Pizza or a cold cheese slice at Little Vinnies (they're famous for this). Walk over to the Book Revue, which is Long Island's largest independent bookstore or catch an arthouse film at the Cinema Arts Centre. You can walk to Heckscher park to stroll around the lake and visit the Heckscher museum of art. You can also catch live music and go bar hopping to experience the local nightlife after hours.
This 409-acre site features an incredible landscape designed by the same firm responsible for creating Central Park: the Olmsted Brothers. Explore over 20 gardens, stroll five miles of woodland trails, delight in the warmth of the two greenhouses, and tour a 65-room Tudor Revival mansion. The park is open everyday from 9am-5pm.
The most obvious tourist attraction is to visit New York City. The best way to get to NYC from Long Island is to take the Long Island Railroad (LIRR) train from the Farmingdale LIRR station to New York Penn Station in Times Square. Duration of the trip one way is one hour. Driving is not recommended as there is sure to be traffic.