LOVED this one. I thought early on, "the one who succeeds is always the youngest," so seeing the Blue Prince leverage that to disqualify his brother made me grin from ear to ear. It was also fun to see him play further games with logic: whoever scotches the beast will marry the princess, whoever the princess marries is the eldest, so turn those back around and the eldest is the one who marries the princess and therefore has scotched the beast. (Or at least that was how I read it, though I also like the explanation that scribes will be busy writing about the royal wedding and not the usual threat.)
And for a story this old, I really appreciated the way the princess was more than a random prize awarded at the end. The Blue Prince won by paying attention to her, and her choice (via pigeon-romance) was what settled the whole matter. She wasn't really a character, but by modern standards none of them really were; they were just types.
I do agree, though, with the people who went "buh?" at the lawn tennis thing. While cute, I don't recall anything in the story that would make that come out of some place closer than far left field.