precipitous (formal), steep
gentle, gradual, slight
long, short
downhill, downward
uphill, upward
higher, upper
There was snow on the higher slopes of the mountain.

lower
northern, north-facing, etc.
The vineyards on the south-facing slopes get more sunshine.

open
forested, grassy, icy, smooth, snow-covered, snowy, wooded
craggy, rocky, scree (esp. BrE), talus (esp. AmE)
mountain
dry (BrE), ski
dry-slope skiing

bunny (AmE), nursery (BrE)
ski lessons on the bunny slope (AmE)
skiing lessons on the nursery slope (BrE)

negative, positive
The unemployment-income curve on the graph has a negative slope.

40-degree, 45-degree, etc.
continental
the continental slope off the American coast

ascend, clamber up, climb, climb up
We clambered up the steep, rocky slope.

clamber down, descend
roll down, tumble down
Rocks and boulders rolled down the slopes of the crater.

have
The field has a slope of about three feet.

hit
She plans to hit the ski slopes this winter.

cover
The vineyard covers the slope.

lead to sth
a slope leading down to the river

rise
The lower slopes rise quite gently.

level off, level out
overlook
a west-facing slope overlooking the river

down a/the slope
I scrambled down the icy slope.

of a/the slope
the steepness of the slope

on a/the slope
We camped on an open mountain slope.

up a/the slope
There were more skiers further up the slope.

slope of
the slope of a hill/mountain/roof
the slope of a curve/line

the bottom of a slope, the foot of a slope, the top of a slope

steeply
The ground slopes away steeply at the back of the house.

gently, slightly
away, down, downwards/downward, up, upwards/upward

to
The field slopes down to a small river.

towards/toward

Slope is used with these nouns as the subject:
roof