If you're stuck for what to write in Mando'a, consider tapping into these resources for inspiration.
"Words by Theme" spreadsheet. Although it only includes words from the original dictionary, it can give you a broad idea of the kinds of things you can talk about with the original language available. (I managed well over 360 distinct sentences with the base dictionary and a handful of fan words.)
https://project-shereshoy.tumblr.com/post/681252496589619200/mandoa-words-by-theme
Grab a randomly generated set of words and try to form one or more sentences from all or some of them, even if they're goofy. Especially if they're goofy, because then you'll associate learning this language with fun.
https://sites.google.com/view/hibirarmandoa/bajur/random-words
Play the Mando'ardle and then craft a sentence using the word that you discover. Just don't spoil the game by letting anyone know that's the word you're using. (To find all words that are five letters long, you can use the regex ^.....$ in the search box. Substitute letters that you know are right for the period in that place.)
https://mandocreator.com/tools/mandoardle.html