The only one I did was meeting my new class. Schools were just reopening after covid! I got paid for the whole day but just went in for the afternoon.
I’ve been for inset days for just mornings and got paid for the whole day.
You could definitely be asked to cover a class.
I did them with my first two years ago and all I did was admin. Sat in the staff room all day and planned/helped create some assessments. Easy days!