Assalamoalaikum Warahmatullah
Regular assemblies are an excellent chance to get the children to memorise surahs etc. becasuse the regularity and frequency definitely help set things in their minds. After a year of daily recitation of Surah Yaa Seen, more than half the class had memorised the entire surah, with some of them moving on to the translation too.
In another school I've worked at, the 99 names of Allah were recited daily (in chunks of 3 at a time) ... this was also beneficial.
I think it's crucial for the assembly to be suited to the age group... i.e. repetitive memorisation and general etiquettes explained simply would be most appropriate for the younger children (Early years, reception and key stage 1)
Reading short sections of classic Islamic texts would be more appropriate for the older KS2 / KS3 age group.
Tapes of stories of the prophets are available in a simple child-friendly format... I think it's a 'Rabab' production - the guys who did the 'Young boy, young girl' series.
These can be played and then questions asked on what was heard.
Hope that's helpful, insha Allah. Sorry for being repetitive at times.
Fee amaanillah
Wassalaam