If you like a bit of spice, feel free to add 1 to 2 minced jalapenos.
You can use fresh basil instead of mint if you prefer.
Choose a watermelon that is heavy for its size that has a creamy orange colored field spot. A white field spot means it is not ripe.
If you aren't a fan of onions pungent taste, try soaking the chopped onions in either water, lemon, or lime juice for around 10-15 minutes to tame down the sharp taste.
You can make this salad up to a dayin advance and store it in the fridge. Hold off until adding the mint and feta until right before serving. You can also chop all the ingredients and store them separately in the refrigerator to prevent all the flavors from seeping together too much (mostly the watermelon and cucumber) as it can become watery the longer it sits.