I understand what you are saying, but pictures would be most helpful, especially to be able to see the spacing of the trees.* It's also important that you understand tree roots before you plant anything else, especially with young trees.* Take a look at these sites.
If your trees are lined up all in a row and you want some color or something that makes it all look cohesive, a mix of shrubs and/or groundcover would probably help with that.* Adding shrubs to the mix will give the area some depth.
You are correct in thinking that it could easily look tacky.* Generally it's best to use no more then 3 groundcovers for a large area like that where the entire site can be seen at a glance.* Otherwise it becomes too busy.* If you just plant a bit of something here and there it tends to look spotty.* In my mind I picture something like the trees just lined up looking like mushrooms that have popped up.* If you remove all grass between the trees and plant your groundcover and/or shrubs and flowers in a continuous sweep, that will give the area some presence.* Eventually the trees will grow larger and offer that presence to you.* Also keep in mind that whatever you plant now for the current sun conditions you have may not thrive as the trees get larger and offer more shade to the area.* Planting groundcovers that run into each other might also become a maintenance nightmare as they merge together.
If you do a mix of trees, shrubs and maybe some flowers and/or groundcover you will have more depth to the plantings and it will look more cohesive.
Here's some pictures to give you ideas.* Here you can see more then one type of groundcover, but it's in large sweeps with only the blue and the yellow blooming at a time so it's not busy with color.
This is a berm of mixed plantings, but you can see there is depth with using the shrubs and different heights and textures of the plants.* Also notice that most groups are in odd numbers of 3 or 5.* Makes it easier on the eyes.
Here's a 'before'
and 'after'.* They did add a little potting shed, but note the plantings between the trees.
Another view of the after from a little further back.
Here's a mix of trees, shrubs and flowers.
Here's a computer generated image, but note the layers that add depth and how the inner plantings are accented by the shrubs on the outer edges.
What do you think?
When weeding, the best way to make sure you are removing a weed and not a valuable plant is to pull on it. If it comes out of the ground easily, it is a valuable plant.