possibly the most dramatic plate-separation-related thing in Iceland I have seen is a point in myvatn right as you are leaving the area to the east (between the lake and the fumaroles over the hill) where the hill is visibly separated down the middle. Take a hill and pull it apart 10 meters in the center. It gets the point across better than larger-scale areas where the rift separation is visible. Thingvellir is too big visually to get the scale of the process.
I’ve just seen pics on TV..it is amazing...when where you there..?