Dianthus (sweet William and others like "cheddar pinks") in general is very good for covering ground at a low level. You may find perennial &/or annual types.
Violets are tough enough to step on, easy to remove if you want to dig a hole for something new, host plant for fritillary butterflies.
Common Blue Violet (Viola sororia)
Or quick temporary plants like Coleus (blooms are beloved by hummers, butterflies, bees - comes in low, creeping varieties &/or easy to trim if it gets too tall).
Or ornamental purslane really covers the ground at a very low level, keeping the ground shaded, cooler, but blooms are only open in the AM. These attract a LOT of bees in my yard, the tiniest cutest little bees I never noticed until I noticed them on these plants.
Wingpod Purslane (Portulaca umbraticola)
Butterflies and bees also love basil blooms, but basil can get pretty tall if you're leaving some stems alone to bloom.
Leguminous annuals like clovers can benefit the soil while benefiting pollinators and your eyes.
Stepping stones eliminate weeds. 12" bricks make great stepping stones, paths, that you can re-arrange at will, pick up to dig in case something does creep under.
Cardboard (corrugated, uncoated - remove any tape or staples, overlap seams, no holes) can also help keep an area clear, or kill existing unwanted growth of non-woody entities. Cover with a few inches of mulch and only you will know it's there. Both cardboard & weeds should be gone by next year. This is a great way to easily expand outward without digging up grass.
