indoors there is an infinite number of possibilities.
for more pronounced ups and downs i use chickenwire, stapled to wooden substructure (some pieces of wood glued down to my fancy)
then covered with toiletpaper (better than kitchenpaper) and plaster. (or white glue, or a mix of white glue and plaster)
or you can put newspapers or cardboxes in a kitchen mixer, cook the result with water to the consistence of porridge and smear that on.
or use a paste of white glue with sawdust.
for rocks, ripp (not cut) apart an old foam mattress, soak the pieces in a glue-plaster-sand mix.
or use thin styropor sheets (1/5")
where you want a hill, put a more or less high “something”, and glue the sheet in the valleys to the underground. (works at an inch height per foot distance max.)
or you make some little crumps of newspaper, and throw an old towel, -wet with plaster- over them.
or take whatever junk your hand can reach without leaving the room.
when using plaster, always “enrich” it with sand or sawdust. - for a rough, more natural surface.
and most important of all… never start landscaping, when you got less than two full pots of white glue.
work from the background to the foreground. - that helps the learning curve and you cover up all the drops of glue and plaster.
and if you are not content with the result, either rip out and redo, or patch up, or take off the glasses, step back and look again.