@frazig,
Where I live, Lantana and Periwinkle are weeds.
Lantana used to try and choke out the citrus trees.
I developed a hate for it spending summers pulling it out by hand.
Someone gave me some periwinkle flowers two years ago, now they are everywhere. I can't get rid of them.
Despite my opinion, they are indeed both beautiful plants. And props to the constant blooms of periwinkle.
Sweet allysum is a nice ground cover. Low growing, sweet smelling blooms.
Doesn't do well in intense heat though.
Creeping basil is great. Spreads at a rapid rate. Blanketing the ground in a sea of green. Smells great and you can use it for cooking.
Pennyroyal, does the same. Lavender colored flowers that look like quarter sized fiber optic pom poms. Has hues of purple in its leaves and covers the bare soil with tiny foliage and a historical smell reminiscent of potpourri.
Both shade the ground, keeping the moisture and coolness in for young trees and considerably cut back on any weed growth.
Marigolds (I think they're weeds too) will carpet the ground in green lacy foliage and forever blooming in orange or yellow blooms that look like dahlias. Their also edible, great with steak or in a salad.
Yarrow. The traditional white cluster of flowers that look like fuller versions of Queen Anne's Lace or you could opt for the newer multi colored clusters that come in red, pink, yellow, various pastels. Spreading by rhizomes, forming tightly packed areas of vibrant green, fern like foliage that looks like asparagus fern from a distance.
Day lilies. Not sure where you are in the world but day lilies grow with abundance where I'm at. Where there is sun, there are day lilies. A bulb that sprouts sword like leaves in a soft fan formation. Blooming spring and summer with large, strong flowers that come in just about every color and pattern imaginable.
Mints or chamomile are nice.
Natures carpet, they are.
Dianthus, or Sweet William are wonderful well forming flowers once established (which doesn't take long).
You could lattice in the areas around your trees and throw down some mulch. That would serve the same desired effect of a ground cover. And you're left with the option of plugging in some flowers or vegetables of your choice.
Lettuce is always satisfying to grow. And serves as both aesthetically pleasing flowers and edible foliage. Vitamin green, winter density, rapeseed, kale (kale is the shiznit, comes in many neat forms - a strong looking superfood), amaranth greens are great for hot weather climates that would normally struggle with lettuces or brassicas. A bonus are their multitudes of different flowering styles.
It really all relies on your climate and zone, the amount of sunlight already available in the chosen locations and the soil. Take those few things into consideration and you can make yourself a low maintenance show stopper. (I just realized why I think so many beautiful things are weeds, the low maintenance aspect. I should embrace that this year.)
Most importantly, have fun with it. (: