If you don't want it to grow taller, then maybe only Bird's Oats (but even that must be cut - otherwise it will grow "wide"). Everything else - if I realize, you have to keep the cut at the chosen height, so choose the right tree according to the habitat (soil type and acidity, shading, altitude, climate) and appearance (deciduous, coniferous ...). Personally, I really like hornbeam (although deciduous, it sheds its leaves only in the spring, when it grows new, grows quickly and is easy to maintain, tolerates even a deep cut) and boxwood is good, shapeable from deciduous trees and conifers (dtto, what hornbeam ). Or - possibly, you can build a pergola and let it be overgrown with climbing vines, sucker, ivy (...), or even a climbing rock, juniper (it is necessary to untie them) ...