On violin you can play a long note, you can change the sound quality while playing, you can change the dynamic like make a crescendo.
On piano there are several options to compensate for that:
A) You can make a trill.
B) You can make a tremolo chord either fast unmeasured tromolo or slower measured tremolo.
C) You can have melodic lines in the right hand with fairly long notes and then add colour to it with arpeggio chords in the left hand. The notes in the right hand would then be played with a stronger attack so the melodic line is emphasized.
Fast repeated notes:
Fast repetition of the same note is easier on a violin compared with piano.
I just realized that you wrote
(solo instrument for the beginning, e.g. violin).
Well, for a beginner violinist don't write long notes. Once they are beyond the total beginner state you can add some long notes, but maybe better keep them "not that long" and not too many.
Avoid C-major. Many violin students start with A-major on the upper strings (A and E strings) but not on the lower strings, like G sharp a half tone below the A string is not for beginners while G-sharp on the E-string (a major third above E) is easy.
At the beginning violin students often play on the upper strings, but it doesn't take that long before they play on the lower strings.
D-major is good on D and A strings. G-major is good on the G and D strings.
Once the students is beyond the total beginner state G- major is very good on all 4 strings. Leave C-major to more advanced students.
For beginners ascending scale patterns are easier than descending scale patterns.
String crossings can be a big issue for beginners. Melodic lines that constantly go back and fourth between two strings can be difficult, but you do need to change strings now and then of course. Changing string to an open string, a nabour string, is fairly easy.
Repetition of notes is great whether you just repeat two quarter notes or make a little rhythmic pattern on a note.
So that is some things that I can think of at the moment.