I feel like most everything should be added at the end of primary to beginning of secondary for ciders. although, unless I am reducing it for a more concentrated flavor, I do not boil my cider. instead I buy pasteurized apple cider with no preservatives added to it (like benzoates or phosphates, citric acid ok), then I can add the apple juice, yeast, and nutrients, aerate, and have the batch done and ready to go in less than 30 minutes
I have had really good luck just placing my fruits and spices directly into the primary and transferring to secondary or a keg when I feel like it has enough flavor in it.
Ginger is a fun one to use for me, I would say if you don't want too "green" of a ginger flavor: peel it, grate it, add honey or thick sugar water to it and keep it in the oven at 350 degrees for 15 minutes, it creates a candy ginger flavor that is very nice in ciders. a teaspoon or two of cayenne pepper (in 5 gallons) helps bring out the apple and ginger flavors too.
for fruits, I would say fresher is better, and whole fruit over puree. You can increase your yield if you have pectic enzyme to break down the fruit first, mango, blackberry, kiwi, and other sweet fruits that don't have too much tartness will be complimentary to the cider as well. steeping time can range from 10-21 days, longer for more subtle fruit flavors. biggest problem here is chance for infection, but if the ABV is above 7.5% its highly unlikely to happen. a phosphoric acid wash (star san) with fruit skins still attached will greatly minimize that risk.
Cinnamon, cloves, nutmeg, and other potent spices should be used very conservatively, a little goes a long way (worst case scenario if you add too much: you have to wait a few months for the flavor to mellow out).
hops are also very interesting to add in a cider, i would recommend a little higher of alcohol and a little sweeter if you want to include that. again, a little goes a long way, .5 oz of 6.6%AA for 5 days is good for a 5 gallon batch. biggest problem here is to have a really fine mesh bag to keep all of the hop particles out of the batch.