If it's for the entire summer, then Kirby's too short.
I'd say either Fire Emblem Awakening or Mario Kart 7.
I don't know what Rune Factory's like so I can't really give advice on that, but I can say that Bravely Default is an okay game but not as good as people hype it up to be. Kinda like all the other Final Fantasy games.
Considering that Fire Emblem is one of the best games out for the 3DS at the moment (and I've played it many times), I personally recommend it. It's truly a fantastic game. Then again, that is just my opinion.
Mario Kart 7 is also a great game. I don't know if you should get it though since Mario Kart 8 just came out.
Fire Emblem - MUST HAVE. One of the best games on the system. Mario Kart 7 - Only fun with friends. Rune Factory 4 - Harvest Moon with dungeons and RPG elements. Great game, ALWAYS something to do. Bravely Default - Best RPG I've played on the system. Fairly long storyline (I'm over 40 hours in and only on Chapter5) Kirby Triple Deluxe - Haven't played yet, but it's Kirby. Cant go wrong!
If you're a fan of Final Fantasy games, I'd say Bravely Default will keep you entertained for a pretty long time. I wouldn't say it's the best from Squeenix, and actually compared to the stories of some of their older games Bravely Default's was a little dull imo, but it's still a pretty fun game.
I would say Fire Emblem is the most all around solid game that you listed.
I found that Bravely Default fizzles out as it is too easy to overgrind and just hit 9999 over and over and over with every character... Overall I simply didn't care for it as much as Fire Emblem
As stated, Kirby isn't that long of a game.
MK7 is always fun and you can play with the majority of users here! That's definitely an added bonus.
I've never played Rune Factory so I can't say anything about it.
Fire Emblem, why? You can marry Tharja. One of the best games on the console.
Kirby TD is quite short.
Rune Factory 4, must have if you love hack and slash rpg + Harvest Moon combined.
Bravely Default is just another FF clone I guess. Not fond of it.