I'll post the picture of your bed below. It might help if you could provide complete links (or better yet, actual pictures) to the things you mentioned that you planned to add. Dimensions and possibly background pictures would also help a lot. We'll need more info like whether your bed is full sun or partial shade. Tell us more about your likes or wants for this bed.

This planter is in full sun all day, and I'm in TN so we get 30-20s in the winter and 90+ in the summer. I'm still new to plants so I don't necessarily have any favorites, but I am a guy so I don't want a ton of pink or anything like that.

Generally, what I would do is...
Move the blocks for the fountain up near the front edge of the top planter. That way I could plant taller things in the back and still always see the fountain. I'd plant a mix of things with varying heights, textures, colors, and flowering times. I'd place taller plants in the back and textures, heights, and flowering times mixed for maximum interest. I always like to use things that flower for a long period of time or have features, like variegation, that adds interest over long periods of time. Most of what I would use would be perennials, because I don't like the work or expense of having to replant each year. I'd probably use some kind of small hardy vine to drape down over the edges of the planters. I'd review each plant selection to be sure it was hardy in my area, not invasive (review the list on TN-EPPC's website), and not prone to significant problems (pest, disease, fussy about drainage, etc).

The way I choose many of my plants is by looking through catalogs or websites of nurseries that I know are top quality. I always review nurseries on Garden Watchdog before even bothering to shop. Sometimes I just log onto Garden Watchdog and choose a nursery out of the Watchdog 30 list to look at.