Hi Kim, can you paint them? Or cover them with a peel and stick wallpaper?
In our last house the main floor walls were made out of cinder block that was covered with paneling. So if you could cover your walls with a type of paneling then you could hang pretty much anything you want.
Paint is the least expensive decorating tool. If your walls have obvious grout lines you could do a faux finish to make the blocks look like sandstone blocks. If this is a rental you can hang fabric dipped in starch right on the walls. You can stretch fabric over large frames (think artist canvas) and use Command hooks to hang them on the walls creating a color block effect or vary the shades of one color for an ombray look. If you own your home consider furring strips and drywall. That option can also extend your access to electrical outlets if your budget allows for the electrical work. If you think the space will benefit from being insulated then consider ridgid foam or deeper studs to accommodate bats.