We have always eaten vegan or vegetarian about two thirds of the time. I was raised that way, so I don't really think about it, but it's amazingly easy and healthy.
So, two things: Look up Mediterranean recipes. They are incredible diverse, and cover everything from Spanish tapas to Lebanese cuisine, and is the healthiest food on the planet.
Secondly, this is good timing, because one of my daughters asked me to send her instructions for something I have been making for years. I'll post it in the other food thread so as not to distract from here, but basically it is something I learned in Tunisia, and makes a perfect healthy replacement for pizza.
One last thing, and this doesn't apply to you because you are doing vegetarian only one day per week, but I have been brewing kombucha continuously for a couple of decades, and it is a good source of B vitamins for people who avoid animal products completely. I drink about a liter a day, and I don't know how much this has to do with it, but I turn 68 in two weeks and I've never even been to a doctor (before people jump on me, I don't recommend this to others, but I just have never had the need). It's a total probiotic and vitamin bomb.