Many times I come across tourist taking advantage of existing tours here. There is nothing wrong with that. It's just that they often miss out on some of the hidden gems that we have and worse yet, often overpay for an experience they could have enjoyed on their own. That being said, a visitor should always use their intelligence and common-sense. The use of reputable tour operators and taxi drivers will eliminate possible problems.
The thing is, St Lucia is an absolutely gorgeous island with hidden gems...well...hidden all over. The typical visitor doesn't get to experience half of them (for that matter the typical local hasn't experienced half of them). We have numerous waterfalls, beaches, rivers and parks/gardens that lend themselves to host amazing picnics or limes*. Most of these are already accessible via the regular tours. Some, due to their distance and 4WD only accessible roads can prove to be a problem to reach. Particularly if you are on a short stay-over.
Renting a vehicle can open up your options and provide an experience at your own pace but driving in St Lucia can be difficult and the challenge may take away from the enjoyment unless you have a true spirit of adventure. That being said hiring a taxi for the day is a good option, though it can prove to be expensive. Whichever route you choose, here my recommendations for "off the beaten track" locations you may enjoy:
1. Dennery Fish Festival - Every Saturday night, the lovely fishing village of Dennery, located on the western side of the island, hosts a fish festival. This festival follows the lead of the more well known Anse la Raye Fish Festival. The difference here is in money saved. Unfortunately the Anse La Raye Festival has become quite the tourist trap and as a result prices have skyrocketed. However, you can still get good deals on the same foods in Dennery. A tip though, befriend a local and get them to order for you if possible. I don't want to say the vendors will try to raise the price on a foreigner but locals get the better deals.
2. Anse La Voute - Also known as the five dollar beach it is located in the north of the island, near the Cas en Bas Beach, this beach has 4WD access and a toll of $5 EC for use of the path made on private property to access the beach. This beach is quiet, beautiful and full of charm. The water is not quite as calm as some of the other beaches but it remains calm enough to be enjoyable. Locals very rarely use this beach, only on long weekends really and I have never encountered foreigners there except those brought by locals.
3. Delmar Bar and Entertainment - Located in the quiet community of Quatre Chemin, Delmar Bar is a gem truly hidden away. This location provides drinks and eats, an inexpensively priced bar, games and a dance floor. Owned and operated by Janique Delmar, expect fantastic stories, good company and the perfect liming environment in a lovely suburban type setting. Karaoke is every Wednesday and Games Night is every Thursday.
4. Camping - Aside from the obvious safety precautions; having a large group, notifying local authorities and possibly getting a local's opinion on the ideal time and places to set up camp, the experience of falling asleep on a beach to the sound of waves and nothing but the night sky above you can hardly be surpassed. Saint Lucia offers many locations that can offer such an experience. It is an experience that locals and tourists alike should not miss out on.
5. Experiencing the country life - Many times locals and tourists alike get caught up in the northern lifestyle of Saint Lucia, ie: Rodney Bay, Gros Islet, etc... There is a lot to be said for the south of the island as well. They may follow a much slower pace but they know how to enjoy nature. The beaches, waterfalls, rivers, back yard cook outs... In the south a lot more imagination goes into their activities. Since less clubs exist people have learned to create their own entertainment and in this way often organise activities that lend itself to greater opportunities to get to know the people and the land around you.
6. SCUBA - and last but not least, Saint Lucia offers an abundance of beautiful dive sites. My words can never do them justice so grab your snorkels or scuba apparatus and get into the water asap...you won't regret it.
Much more exists but these 6 should get you started. Remember to walk the path less travelled and enjoy the sights few have seen.