Leliana's Song has nothing to do with the main plot... really. So you can do it any time. Golems is FREAKING HARD, so I did that last, and also has practically nothing to do with the plot. The only one that has any bearing story wise (and it's negligible) is witch hunt.

I did Leliana's song, then Witch Hunt, then Golems. I think Bioware intended for Witch hunt to go last, but it was so much easier than Golems... *shrug*