Suggest you repost your question on the Perennials Forum where you should receive some helpful suggestions. I am not certain of your growing zone but Bergenia and primoses might be good filler plants to consider. Also, almost unknown is the fact that peony foliage can be cut back to the ground with no harm to next year's growth about the end of July. I have often used this method in my perennial garden designs to fill in blank spaces while waiting for fall blooming perennials to shape up.

Whatever you choose -- some would say sand, some gravel -- use a plate compactor for every 6" or so of lift. In addition, I would drill around the perimeter and epoxy in rebar stubs that can be tied to rebar pieces spanning the new slab. It looks like you have a lip around the top edge, and that will help with supporting the new slab.

Thanks to you all for the compassion and ideas! You guys are great. I put on my big-girl pants and took care of business. The hole had a big right-hand turn so I wasn't sure a broom handle would do the job. While holding my breath, I poked around deep in the hole with a garden hose. Either nothing was in there or nothing came out. Thank goodness. I filled the dirt back into the hole, packed it in hard, covered it with another stepping stone, and that was that. Whew. It's funny that four of my stepping stones are now relocated away from their usual part of the path. I will recover them later. Writersblock, i laughed at your unpaved ground comment. This makes me want to put a layer of cement all the way around the house. But I'm sure Mr. Hole-Digger would just start digging outside of that perimeter. Carol

The material to use is Vinyl Concrete Patch powder. It's available at all the big box home centers. You can also use hydraulic cement, which is better if you have a high water table and problems with water seeping through the holes. Get as much dirt out of the holes as you can with a vacuum cleaner. If possible, undercut each hole a little with a chisel (make the holes larger at the bottom), but if you can't, don't sweat it. Mix the compound as directed on the container. Don't use too much water. Dampen the inside of the hole with water from a spray bottle. Damp, not sopping. Stuff the compound in and smooth it with a putty knife. Let set. Done.