I am going against the grain here to suggest a simple, cheap sun shade from Amazon (sometimes called sun sail or shade sail). I have one I use over the back deck of the boat. When it fails I'll buy another cheap one.
Obviously it needs to hook onto something, and if that violates the "no attachment" rule then you'd have to get creative. They generally have grommets on the corners so you can put them on a pole. That's what I do on the aft end of mine. I have a pair of adjustable poles I stick in a couple of well-placed rod holders. Presumably you could do the same with planters or whatever.
Think of it like an umbrella with the poles on the outside corners, instead of the center.
I was never a fan of umbrellas for shade. They only shade a small patch at a time, and it moves over the course of a day. At lunchtime they only cover the table, not most of the seats around the table. In the morning or evening when the sun is lower they're even less helpful. I don't enjoy sitting in the sun.