I'm not sure which EP to get with Ambitions, Late Night and Outdoor Living. I like the idea of owning a horse, but I also want to have my Sims travel and be able to see the kids graduate from high school. What EP should I get?

Get Pets.

Generations is really only worth it if you get it on sale. World Adventures is already cheap, but it comes second to Pets IMO. If it was between World Adventures and Generations I say go with whichever is cheaper.

Since Pets isn't out and I have no idea how good it'll be (for all I know it will completely break my game, as it always happens with brand-new EPs), I'd put Pets on the same spot as World Ambitions. And, meh, why not get both? :D Since it's your b-day, maybe you could ask for a bigger gift? Or maybe get one for your b-day and one as a very early Christmas gift.

But Generations isn't really worth it, unless you find it at a terribly low price. It does bring along a few more interactions, but overall it's not as consistent as other EPs, especially since about half of it was released with a patch (memories and reputation).