Paint or plaster over them with white. Your ideabook photos clearly show white everything.

You don't mention what design style you are trying to accomplish. In any event I'd recommend painting them in white or off-white in their current rough condition - this could be quite attractive. Then, as you suggest, if you are not happy with the rough appearance they could be plastered for a more clean and contemporary appearance. Meanwhile, you should consider how you are going to light this room - ie will you be able to install any light fixtures between the beams? Is the entire ceiling between the beams concrete as well? In which case how would you mount the light fixtures and how would you get power to them? If it's impossible to get light fixtures in the ceiling then you will need to rely on indirect lighting somehow.

Hi - I'd like an industrial/Scandinavian type look. Good point re the lighting. I need to double check what's in between. We're very much at the beginning of the project. The whole building is being renovated - not just my apartment.

With industrial / Scandinavian look the painted beams would be appropriate. If the ceilings between the beams turn out to be concrete, you could drop drywall ceilings between the beams in those areas to conceal wiring for light fixtures. Surface fixtures would require dropping the ceiling just a few inches. Recessed LED fixtures might require 4" - 6" dropped ceilings between the beams. You'd then would need to figure out a way to get wiring to those locations - which would probably mean drilling horizontally through a beam or two somewhere if a structural engineer OK'd it. I like the exposed concrete beams in any case as they will give visual interest to the space, and also give a loftier feel. It would be unfortunate to conceal them with a suspended ceiling.

Thanks so much for your answer. I will speak the builders and see what is possible.