Giant tardigrades would have been fun. Serious, authentic space travelers... not so much on the cuteness factor, though.
Like, you might have a aquatic space-faring species, which requires you to bring lots of water into space. More rockets (and struts!) will be needed, but at least you get really good radiation protection out of it!
Tardigrades ought to be a cheat code, you can just glue your crew to the outside of the booster and they'll probably be alright so long as you don't glue them to the fiery end!