Christian, scenery is not scary, there is any number of successful ways to go, and you can correct or dull down, so mistakes can be addressed, flat earth syndrome on a large layout does not apply to small layouts so that is not an issue either, you can even look at magazines for backgrounds if you just like greenery, here are some of my mistakes, and successes, hopefully they might help you jump in
Early on I painted the surface green, then used dayglo green grass - not a good look in my opinion, and I love autumnal trees but those fall colours are the hardest to look righ, bare winter branches or spring is much easier.
ad I developed I found an earth colour , terracotta works well is a much easier way then add a dull grass, I have tried static grass in lots of forms, including an electronic applicator, yes it is the best - but also comes with a hefty price tag, a puffer bottle costs less - and frankly any plastic bottle can provide static rub it with a dry cloth see if cling film sticks if so you have built up sufficient static, and even shaken from a piece of paper slowly in small loads can make acceptable finish, or pinched into glue making little clumps, purposely leaving some earth colour bare works well and you can use mud but cover it with glue as if Ballasting track, and mix the colour a little with black or white so it is not flat but has shades, if a hole go with black for a little shadow.
if adding grass by hand rather than with a more expensive option try different textures the mix helps hide the fact it is hand laid,and in fact often ends up with a pretty natural finish
if you have little confidence in your skills for a back ground just go pale powder blue sky, it always works.
the picture above has static grass and wood grass, if you can get your hand on some wood shavings (from a pencil works) crush the paint dip in green paint, and preferably in three shades like camophlage, then dry on tissue paper, even the tissue paper can be used
with thick paint and some polyfilla I āpainted the back scene low, painted brown, then pinched wood grass on the surface to infer distant hills
photographed only a short distance back from a better angle, the shrubs in front are horsehair rubbed with glue and added wood shavings painted, also dried moss from the garden works well
if you like the ideas and want more shout out.