Trees are not a given. You could have a world whose only plants are grasses, or at least grasslike. Evolution on Earth favoured large woody structures in land plants but that doesn't necessarily mean that that is the only viable pathway. It is plausible that land plants could have maintained the frond like structure of the Caulerpa seaweeds which resemble underwater grasses, albeit of potentially very large size. The problem then is simply one of tuning the world temperature and water balance to the point where there are no polar caps etc... to interrupt the spread of these primitive grass-ish plants.
All the land is mountainous and effectively at high altitude is easy but that means having a lot of water and a thin atmosphere. The world is covered in huge oceans divided by narrow mountain chains created by uplift along tectonic collision zones, the thin atmosphere makes sea level feel like 13,000 feet. To have a world with no ocean and all mountains you need gravity and lots of it, the world itself will be relatively small but it will be orbiting close to it's Roche Limit in 2:1 Resonance with another large world that also gets close enough to cause land tides that are a noticeable percentage of the thickness of the crust. This world will undergo continuous and massive crustal stresses that crack apart and mash together its crust creating a surface covered in ridges and valleys that look to an Earthling like mountainous terrain.