Tomatoes: I use cherry tomatoes in this salad because I like them better and they usually aren't so seedy like large tomatoes. If you're using large tomatoes, remove the seeds first and then dice them.
Cucumber: I highly recommend seeding the cucumbers before slicing them. Cut the cucumber in half, then split each half lengthwise (cutting it in half first makes it easier to split it). Scrape out the seedy part with a teaspoon and discard, then dice the now seed-free cucumber.
Yellow pepper: I choose to use yellow here because there's roasted red pepper, also. If you prefer red peppers, feel free to use a red pepper in place of the yellow. And check out how to cut a bell pepper like a pro!
Olives: Feel free to use green olives if you prefer them. My favorites are Kalamata olives in this recipe. You can also just leave them out if you're not a fan of olives in general.
Parsley: If you can get your hands on fresh oregano, I highly recommend using that in place of the parsley. I only have access to fresh oregano when I grow it myself, which is hardly ever, so parsley is definitely a much more accessible ingredients to use here.
It's excellent as an easy make ahead/meal prep lunch: Just store it in a clean, well-closed container in the fridge for up to 3 days.