I had always thought I wanted stones on top, I had ideas of buying some large decorative stones but they are expensive, and I didn't think they would suit the wall, or the wall wouldn't suit the stones! I have quite a few small stones I have dug from the garden but I do use the top of the wall to stand a lot of pots on, so I have changed my mind on that.
A seat is also an idea I had, if I can find some suitable wood I may do that in one spot as I often feel like I could do to sit for a while.
I noted the mention of putting soil around a tree trunk, my tree was quite depleted of soil around it, and the mixture is shallow with plenty of leaves. Some tree roots have grown up into it so it probably appreciated the top up.
I have a photo of it in in full early spring before most things have grown, but you get the pic....