Try something like copying out the score of some piece you like. This will allow you to see how the composer put things together. (In the past say from 800 to 1800 it was a common method of teaching.) If you have a computer music program (Finale, Sibelius, etc. or Finale Notepad, the free one), you can try scores for ensembles of instruments that you do not play.
I used the free Finale Notepad for years then Make Music offered me a student upgrade to the full Finale for a small amount. I imagine that other companies have similar offers.)
Another idea is to work out some counterpoint or harmony exercises designed for teaching. Pick a few exercises from various books (different writers emphasize different aspects of music) and give them a try. You can see how Bach puts melodies together or how Beethoven structures sonata or the like.
One more thing to erode writer's block is to write short pieces exhibiting a new (for you) idea. My next couple of short pieces consists of trying to use a secondary theme (sonata or next part of a song or other part of rondo or the like) using Neapolitan harmonies. (I'm copying Beethoven and others here). I want to use something like C-major or c-minor (or whatever keynote sounds good) for the main theme then Db-major for the secondary theme (as opposed to G-major or Eb-major or the other mode of main key). The idea is to make it sound convincing.
Similarly, one can write a piece in an odd time signature (5/4); one may not do this for "real" but it makes a good exercise on phrasing. One could try a new riff or motif; or new accompaniment style. Sometimes I try writing for a new instrument. (I even composed a duo for bongos and congas to get an idea of how these instruments combine.)
The idea is to try something new. On wants something limited to keep from being overwhelmed. Also something that's fun.