In my dd's nursery there are 18 children (mornings; similar afternoons I think) with one teacher and one nursery nurse. I don't know exactly how their 2.5 hours are structured, but I know that they have 'carpet time' when they go in, a certain amount of free play, but also snacks/milk, outdoor play (in two groups of 9), and they have 'themes' that they will talk about/work on for a week or so (e.g. about holidays, or the weather, or shapes, etc.). They have just recently started doing a 'letter of the week'. So it might be 'b' and they will practice writing (tracing) it, find objects beginning with it, etc.
At this school they tend to move into the Reception class part-time once they are 4, though (and go full-time in the term in which they are 5) and there the organisation is more structured and 'school-like'.
It is all quite unstructured, but I am fairly certain that there is research that shows that 3/4 year olds learn best in this sort of unstructured way. I'm no expert, but I have a friend who is the Head of a nursery school (Beacon School, loads of prizes and accolades, classed as outstanding by Ofsted... but far away from where I now live, sadly!) who is very insistent on the fact that the kids should pretty much have free choice of what they do. She says that some will choose to do, say, painting every day for the first week or two, but they soon get bored and want to try a greater variety of activities. But she would never force them, say, to 'come and do some counting' (or whatever).
I hope this helps.