Canned lentils work better if you rinse them really well before making the salad, because that gets rid of the sodium and that slightly slimy texture they have. Once you've dried them thoroughly (paper towels work great for this), add a strong vinegar - red wine vinegar or balsamic - and let them soak for about half an hour. That's what really brings them to life, because they absorb the flavor and lose that flat taste they have fresh out of the can.
For the rest, throw together whatever you have on hand: tomato, cucumber, some red onion if you can handle the bite, cheese (feta is perfect, but even regular semi-cured works). What nobody usually mentions is that a handful of raisins or dried cranberries gives it a sweet touch that contrasts really well with the acidity of the vinaigrette. And if you want it not to be boring, add something crunchy - nuts, toasted bread, whatever you've got. I love letting it sit in the fridge for a bit before eating; the lentils get more body and the salad becomes way more flavorful than if you eat it right after making it.