I think you nailed it! That's what I would do....either that or a gift card to the local farm store, but it's not as personal as the bucket. Wow if someone gave me a gift bucket right now, I would be so happy, I wouldn't care if I already had 5 brushes, 2 bottles of shampoo, 2 bags of ties, 5 halter, ect lol horsey stuff is so much fun!
I think your idea is great! I'm not real artsy so if it were me I'd probably fill up a big bucket or caddy with stuff (example Browsing KBC International Inc.- Tack Caddy Box) and then wrap it in some sort of cellophane stuff and decorate that with ribbon and stickers maybe. It'd be fun!
The bucket's a good idea! Other ideas are polos with her favourite print (sponge bob? Plaid? Teddy bears? I'm not here to judge lol), a personalized license plate for the front of her car if she drives - with a picture of her horse and its name, an educational book (maybe she wants to learn more about something?), a sign for her horse's stall, a service or a whole book of them (stall cleaning, clipping, feed tub washing, tack cleaning...)
If you go to the tack shop you'll see all kinds of ideas forming in your head! I am easily inspired :)