Algunos vs unos.

Hola Iain

The use of "algunos" there would be a bit unnatural but is a difficult one to explain. We often tend to use "unos, unas ..." when we see that plural noun as a "unified plural" as opposed to a group of things seen independently. In this case, when saying that "they sat on some benches" we see the benches as a "unified plural".  Here's a very extensive article form another source where you can see how many different and subtle differences there are between unos and algunos. 

unos vs algunos

As for "¡Qué espectacular!, yes, you can say that to express pretty much the same thing as "¡Era espectacular! - With qué it'd be more precise to translate it as "How spectacular!, but the idea is nearly the same. 

Saludos cordiales

Inma