How do I wire up a shed for 60A service today with the ability to grow later?
Subpanel

I'm glad you got a decently large panel because you don't want to know what a CH duplex breaker costs.  CH products are great and sturdy, and I would use them everywhere, but the deal-killer is exotic breakers are wildly overpriced. Other people make great panels too. 

You're making a classic blunder by worrying about the size of the main breaker in the subpanel.  It doesn't matter; it's nothing but an on-off switch.  Whatever practical reason you might imagine, isn't going to work the way you want.  Particularly, you can't get the local breaker to trip first, doesn't work. 

So just go ahead and do what's cheapest, e.g. keep the 100 it's supplied with.  Its only purpose is a shut-off switch (and GFCI or AFCI if it does that, but you can't backfeed those.) 

Gory details?  OK.  The breaker in the main service panel protects the wiring to the subpanel.  The main breaker in the subpanel protects the subpanel itself, but that is almost never an issue and won't be here.  

Conduit and wire

You upsized the panel, good plan. You're going conduit, good plan.  Upsizing the conduit above minimums, also good plan: it makes pulling easier and lends future expansion.  Easy pulling is vital for DIY, so you can avoid hiring an electrician simply for his truck full of pulling tools. Electricians won't like doing just the pull and will want a bigger bite of the whole job.  

Try to keep bends to a bare minimum, as they make pulling much more difficult. I aim for a maximum of one 90 degree bend between access points (boxes, conduit bodies etc.) but that is not possible in an outdoor run.

"Cable" describes a bundle which contains more than one wire.  You don't need to use cable in conduit; it's redundant and a waste of money (unless it's not, best to price it all ways).  You can use individual wires such as THWN-2.   You can use all-black if you want; wires 4 AWG or larger can be taped to indicate neutral and ground.  Wires are easier to pull than cable. 

Aluminum wire is perfect for this.  You're absolutely welcome to oversize the wire (use a greater ampacity than you're breakered for).   If it won't fit on the breaker or lug, pigtail it with a screwdown splice.  

As of NEC 2017 you will need to use a torque wrench/driver to set torque on screwdown connections.  So hurry :)