It looks like it would make an awesome base for a sculpture!

Would you like to make something yourself or hire your neighborhood tinkerer/sculptor to make one?

I'd think METAL, and for me, it would have to be reminiscent of Alexander Calder. That is, something mobile, but not too light weight or a severe wind would tear it apart.

This is one of his from Stuttgart Germany, what do you think? Yours could be modest but very cool too. (Wow -- it's only 6 KB and a very grainy image!)

Gus

It may some help to plant hosta,fern and some flower on other place.

If you don't want to just mulch it try planting golden creeping jenny moneywort around the trees. Plant the rest with shade loving annuals like impatiens or begonias or coleus or plant perennials like hosta and ferns and where it gets a bit more sun day lilies and bleeding heart luxuriant and anabelle hydrangea will do ok. Hosta comes in lots of shades from purple to lime green to white and every shade in between as well as different heights

I have a hosta about 3 feet from a large fully grown 40 foot maple. It took a couple of years to get established because of the tree roots but it is huge now. I also have golden creeping jenny about a foot from that tree and it thrives there and insists on creeping closer to the tree. It's bright yellow-- almost lime green leaves really pop in the shade. It could become invasive if you let it but fortunately it is easy to keep in check. It is very shallow rooted and grabbing a handful and tugging breaks off runners and sometimes brings up roots with little effort. I think it's shallow roots are what allows it to exist with trees. It's drought tolerant too.

You could try adding something like a bird bath in that space too.

I hope I've given you some ideas

Do you need it to be a bed? Could you just let it blend in with the grass? The boulder could still stay, just like it would in a natural landscape.

"I would like to do some form of rock but ... " Do you mean stone mulch ... like a surface LAYER of stone? I would so NOT do that. It would look like a scab on the earth and be difficult and expensive to undo at some later point. Or do you mean some form of stone EDGING? While this can be done nicely, it almost never is. Most versions are a cheap knockoff. Why not just go with a solid bed of groundcover? (... not one that is merely ringed with a row of plants at its outer perimeter, but one that is planted solid and full.)