Prints, mirrors, and oil paintings come first to mind, but too many of these can be boring. Try to consider with: Wall brackets, Sconce, Plates, Trays, Swags, Tapestries, Architectural pieces, Tall plants, and Pedestals.
Wall brackets (especially curved ones) and sconce not only add dimension but also offer relief from the hard edges of frames. You can add a variety of accessories atop your brackets. Try urns, plates, statues, small oil paintings, floral arrangements and topiarist. Brackets are especially good flanking a china cabinet, door entry or a picture on a long wall. They can add importance and help "s t r e t c h" a mirror or picture on a long wall and achieve a more balanced look. A single bracket is also a good solution for treating a narrow wall.
Plates also offer dimension and softness by virtue of their shape. They are wonderful "double stacked" or "triple stacked" in a narrow space. A collection of plates can be scattered effectively on a wall. Odd numbers (3,5,7) typically work best.
Trays are great above a doorway when there is enough space. The round and oval trays are better than rectangular ones because they offer softness to the squareness of a doorway.
Swags are another alternative. However they are seldom enough alone unless they are used over something, such as doorways, pictures, mirrors
or tall headboards. Over a small picture or mirror they can be used to stretch the look vertically where you are perhaps using a shorter picture that the wall really needs.
Tapestries are a wonderful addition to any room. They offer texture on your walls that can be achieved few other ways. If they are hung on a pole rod they also offer depth.
Architectural pieces - pediments, shards, plaques and overdoors typically add interest when used above something. Medallions and plaques are better used alone or over something larger.
Hanging something on your wall is not the only way to accessorize them. Tall plants cover wall space and add softness and dimension. Putting a plant, statue, or floral arrangement on a pedestal achieves the same thing. A folding screen in a corner is an option that should not be forgotten.
The secret to effectively using all the different options is variety in placement. You can achieve a sense of balance by alternating vertical and horizontal wall décor. Two horizontal pictures "double stacked" can achieve a vertical look if they cover more height than width. You also need to separate your pieces with hard edges with those things that offer softness and dimension.