Some points to note.
Rivers flow where they do because of elevation changes that much is obvious, but the part that is often missed is the "path of least resistance" issue. Water will run down a mountain (especially if a glacial peak or seasonally iced/snowed peak) to river at the base. But the river wont flow away from the mountain until it has traveled along the base of the mountain for a ways. The weight of the water is not enough for it to force a new path yet, so it follows the easy path where the stone of the mountain turns to softer earth (usually with a layer of stone just beneath preventing further draining).

All mountains are surrounded by foothills. Usually steep sided hills which flow at uneven elevations into one another. While that is true in reality, if you put them all on a map it quickly becomes overwhelmingly busy, and the hills dominate what the viewer percieves. Place your larger foothills deliberately, and leave the lesser ones implied, dont actually render every part of the range.

Regardless of climate, broadleaf deciduous plants require a lot more water than conifers (needle leaved plants like pines). And conifers survive well in colder climates. So even in south american jungles, as altitude increases the amount of broadleaf plants decreases, then conifers begin to dominate. As the altitude decreases, the broadleaved ones become dominant, and thick and heavy near lakes and streams.

Grasslands tend to show up
1)where water and temperature dont allow for broad leafed plants, but winds and weather dont allow for conifer seeds to take hold. (High Plateaus)
2)The overall weather is warm enough for plants, but the growing season is too short for trees to gain a measure of maturity before the snow flies. (Think Siberia. Yes there are forests there, adn big ones at that, but the tree types are all stunted pines and large shrubs)
3) Where there just isnt enough water for trees (African savannahs)

You said you wanted some realism Hope it helps.