Though St. John is known as one of the more pricey islands in the Caribbean, it is surprisingly easy to NOT spend a lot of money while you're there, provided you avoid shopping and eating out. Though fine dining and duty-free shopping ARE two of the most popular pastimes on the island, there are some low and no-cost options activities too, especially when it comes to outdoor recreation.
Hawksnest Bay — Photo courtesy of Karen ElowittFirst thing in the morning, grab a low-cost breakfast sandwich at Cafe Livin' (about $5), then head to one of the gorgeous North Shore beaches. Each one of this string of beaches offers unbelievably powdery white sand, absurdly clear water, and an abundance of watersports. For beginning snorkelers Trunk Bay is a favorite, as it features an underwater snorkeling trail around a small offshore cay. Cinnamon Bay is also a good option, because it has a little something for everyone: snorkeling, hiking on the marked nature trail, plantation ruins, windsurfing, and a beach bar. But Trunk Bay and Cinnamon Bay charge a $4 fee per visitor, so if you want to REALLY cheap out, try Hawksnest Bay or Maho Bay. Both are free, but don't feature amenities, watersports, or bars the way the others do.
the outdoor patio at Miss Lucy's — Photo courtesy of Karen ElowittFor lunch, head over the hill, past Coral Bay, to Miss Lucy's, a favorite island eatery. Located on a gorgeous cove facing Friis Bay, the menu includes both American and West Indian favorites such as spicy callaloo stew, conch fritters, and kettle-cooked paella with hot sausage, chicken, shrimp and mussels over saffron rice for $10-$15 each. If you happen to be visiting St. John on a Sunday, don't miss the Sunday jazz brunch.
In the afternoon, choose one of three options:
1) Spend a day at one of the spectacular beaches on the south end of the island, such as Salt Pond Bay beach or Lameshur Bay beach. Both are quite the opposite from the white sand, picture-postcard beaches of the north shore, but instead they offer much more seclusion, and much better snorkeling opportunities (think sea turtles!)
2) Hike the Reef Bay Trail, which descends 2.5 miles through jungle and past ruins from the highest point on the island to the southwestern shore. Make an appointment for a guided tour with the National Park Service and you'll take a boat back to Cruz Bay instead of having to hike back uphill to the starting point. 
Love City Beach Shop, in Cruz bay — Photo courtesy of Karen Elowitt3) Spend the afternoon browsing through the cute shops and art galleries in Cruz Bay. The Mongoose Junction shopping center offers a slew of fabulous boutiques and galleries such as Bamboula, Bajo El Sol, Bougainvillea, the Fabric Mill, and R.I. Patton Goldsmithing. Down in the town center you'll find the Pink Papaya, St. John Editions, and Now and Zen, in addition to numerous other boutiques offering distinctive jewelry, unique crafts, and colorful clothing. Window shopping is free, and will let you soak in the barefoot-casual atmosphere of Cruz Bay.
In the evening, head to Woody's Seafood Saloon for $1 beers during happy hour (3-6pm). Stay for dinner and indulge in some of the inexpensive appetizers and entrees such as conch fritters, blackened shark, drunken shellfish, or mussels and clams steamed in beer (about $10).