Scallions are a great idea. If your people like raw red onions, they can be nice. (Just make sure you get one that's really fresh or it can have a slightly unpleasant odor.)
For a crunch element, how about candied almonds?
Thinly sliced green apples.
Thinly sliced fennel bulb.
Fresh herbs of some sort can be nice if you have them. I like dill and cilantro. (Cilantro makes some people angry. Use with care.) Tarragon plays very well with carrots. I'd stay away from woody herbs like rosemary and sage.
I also suggest adding a bit of something spicy. A splash of hot sauce can be very nice in vinaigrette.
I like wilted salads now and then. You just heat the dressing and pour it over. If you put the mushrooms into the dressing and let it simmer, they cook a bit and get a nice marinated texture.
Grate the carrots, for sure. Partially peel the cucumbers (four strips each of peel and no peel?), then slice. As for putting it together, I think it depends on whether you are plating individually or in a large bowl.