Next time I do cider, when I keg I'll add potassium sorbate, and then sweeten it to taste. Then force carbonate. It's just to dry for me without some back-sweetening.
As far as adding to the recipe goes, I'd use extract before brown sugar for bumping up the alcohol content. Brown sugar is just cane sugar with molasses added to it. So it will give that tang associated with cane sugar, which I don't like. Personally I don't like all-extract brew, but we're only talking a few pounds here, not the entire recipe.
I did that, actually. My last batch of cider. Added the last of the extract I had left from before I went all-grain. Dissolved on the stove in some of the cider. Came out OK. Like I said, it's too dry for my taste. In the end I was putting some simple syrup into the glass before pouring in some dry carbonated cider from the keg. That wasn't bad.
Spices... I would think that by virtue of not being boiled, they'll impart less flavor. Maybe a cinnamon stick and a few cloves? A few allspice berries? A pouch of mulled cider spices? Citrus zest?
By the way, I used Safale US-05. From what I've read since, yeast marketed for cider make a better product.