In a small saucepan, bring the water and a pinch of salt to a boil. Add the quinoa, cover, and simmer for 15 minutes, or until the water is absorbed. Transfer to a medium bowl to cool to room temperature, then fluff the quinoa.
Refrigerate the quinoa for 20 minutes.
Add the spinach, cucumber, tomatoes, and avocado to the bowl of quinoa and mix to combine.
Add the lemon juice, olive oil, salt, and pepper, and mix well.