I've been doing the same thing, a head of romaine lettuce, a handful of cherry tomatoes, a thinly sliced 1/2 cucumber. For the dressing I've been using the juice of one lemon, I don't have a food processor, so I use a coffee bean grinder to grind up some nuts into paste (I've tried pine nuts, almonds and walnuts so far) add that to the lemon juice and add a bit of water to dilute it to taste.
The almonds took several bouts to grind them up, the pine nuts were more of a sticky paste. I think the walnuts turned out the best.
I nicked the recipe off a YouTube video, I think it was a Megan Elizabeth video.
You could try 1/2 lemon and 1/2 orange instead of a whole lemon.
my favorite= half avocado, stalk of celery, some green onion, juice of one lime or lemon.