Surprise your audience with your characters. You might be tempted to make your lead character a strong swashbuckler or a character like Indiana Jones. Instead, create characters that are unlikely heroes. Children or a disabled protagonist are examples of this. These characters are likely to draw more empathy from your readers.
Don't spend too much time on the internal workings of your characters or delve too deeply into their back stories. You want to keep the writing in an adventure book external and allow the adventure to be the star of the novel. An adventure book should be about the adventure. The characters can't be one-dimensional, but their lives can't overshadow the adventure.
Write with a quest in mind. Adventures are usually built around searching for something, whether it's a lost treasure or a cure for a disease. If there's a quest, the story that arises around it will most likely be adventurous.
Include plenty of action. The terms action and adventure are often used together. This is because adventure novels should be full of action. Action is what compels the adventure. This doesn't mean you should not write good dialogue, but the dialogue isn't where you want the focus of your novel to be. Your book should depend on intense action, with every scene written to take your characters another step toward the story goal.