Very entertaining game This game reminds me, very much, of the Bionicle Toa Mata Platform Game from the Toa Mata Nestlé Promo CD - a promotional item found in many Nestlé cereal boxes back in 2001. I've had the pleasure of playing that game when I was much younger. The similarities are unmistakable: the scrolling platforms, the increasing difficulty as you get to the later stages of the game. It's probably just a coincidence though.
I commend you on how charming this game is. I especially like your choice of music and the cute visuals (even the bloody splatter when you plummet to your inevitable death or hit a thorn is kind of cute). I also like how the platforms are randomised. It seems people don't like this but everyone would be bored to death if all the same platforms appear each time. It is unfair sometimes (I got a high score of 99 on my second play-through after missing an impossible jump, for example) but that's just the way it is. I have no problems with that.
I will raise some points that I found annoying - mostly the balloons. It's quite frustrating when you're on a platform and you don't really want to step on a balloon but you bounce of it anyway. I thought, at first, that the balloons were either poppable things (they weren't; stomping on them multiple times didn't do anything) or something you could use to help you, which is true in some instances but for the most part they were a nuisance bouncing you around when you don't want them to. I'm not saying get rid of the balloons; they're part of the game's charm, but I do hope they weren't so annoying and bouncy when the dog's paws are already on a platform. Nonetheless, this wasn't really such a huge hamper to the game's enjoyment.
I don't really have too many negative things to say about this game. It's plays like one of those viral mobile games - simple and easy but not really too interesting; the only thing keeping you playing is the prospect of beating your high score. And for that reason I agree with CyberDevil that you could add a leaderboard.
Another suggestion I have in mind is to have unlockables, similar to what theplayer234 said. New backgrounds would work really well. Maybe the platforms could be made to look different too, e.g. snow instead of grass and sharp icicles instead of thorns. I remember the Bionicle game I played had unlockable backstory cutscenes for each character when you collected all the things you had to and finished their part of the game... if I remember correctly. I'm thinking too much here, but you get my point. It's fun as it is but adding more stuff would make it better.
To be honest though, I got bored after I got 100+ and I didn't want to play anymore. I know this wasn't really meant for me, or anyone else here anyway, so I understand if you don't want to bother with a leaderboard or future modifications. As a gift for your girlfriend this works really well. It's simple, cute and fun. It doesn't really matter that it's imperfect. It's great for what it is. Plus, you always make really good games so godspeed to you!