I see what you are saying but, if they are doing a beach that day, it is helpful for one of the members of staff to show the techniques to create a beach picture.
Having had a child in two years in a nursery setting where there was no direction whatsoever other than handing my child a pot of pencils or a pot of paint, simply leaving them to it does not work at this stage. We had two years of brown splodge. Even my DD did not know what she had painted. She did not learn techniques such as drawing animals, people, buildings or anything else. She was not painting or being creative being left to it. She was just putting paint on paper.
I agree that some school/nursery pictures are a little generic so maybe there could be more 'ideas' put on the table - so, instead of yellow suns, there could be sunsets, too, or maybe pencils where they could do their own sun and the teacher could stress that they don't have to do it in a certain way if they want to do something different. Perhaps there could also be some ideas of what to put on the beach and it to be stressed that they could also create their own stuff with the materials provided (inc paint and pencils) or don't have to use them all.
On some days, there could be the option of paint/create anything you want, with perhaps some help/guidance, if needed, too. So, if they want to draw a cat or their Mummy, perhaps the leader could show them some tried and tested techniques to do that. This really helped my daughter grow in confidence so that she did go on to develop her own style later. We did not know she had artistic ability until she'd had guidance.
I don't think there's anything wrong with 'today, we're drawing a beach' because they do this with English - e.g. today, we're writing about what we did in the holidays.
Sometimes, when prevented with too much choice - i.e. write about whatever you like - children become overwhelmed and then stuck and lose confidence because they don't know what to do. I think I was like this as a child.
As for more conceptual, abstract techniques, they did teach both of mine this at primary - e.g. based on Picasso, Andy Warhol, which they really enjoyed. However, even conceptual artists learnt traditional techniques and then went on to find their own niche so I don't think it would necessarily stifle their creativity.