Bac Ha is a three-hour drive from Sa Pa, which makes it a very long day trip. A more usual and sensible arrangement is to go straight back to the train after Bac Ha. But the market is colourful and fun, even if you don't want to buy a horse or a buffalo or a ploughshare. We bought bright things like scarves to take home, cheaper than we saw anywhere else.
There are interesting things to see and photograph on the way; we got the driver to stop just once (to take that photograph of a buffalo in a rice field), and I got one lucky shot through the window on the move, but if we'd had more time, and if I'd been more assertive or could speak Vietnamese, we would have stopped many times.