Yes this is our first cruise. Regarding booking activities, I was just able to do that a couple of days ago (75 days out as a newbie cruiser). I was actually very disappointed with that process. My daughter is completely princess-obsessed, which is why we booked a
Disney cruise in the first place. But the Princess Gathering and the Frozen meet & greet were already completely sold out at midnight at 75 days, so first time cruisers didn't have a chance. I knew this happened with Cabanas at Castaway and Palo, etc, but I did not really think the Princess and Frozen would be unavailable to us. So, yeah, supremely disappointed.
I was able to book activities that cost extra, such as BBB and the Royal Tea, which is crazy $$$, but I booked it anyway as my only way to guarantee princess time. Apparently they hold back some Princess and Frozen tickets for after boarding, and I will still try for those, but that will be too late to cancel the Tea if I am able to get them.
I did not try for Palo/Remy or any spa activities, so I can't speak to the availability of those. Excursions still seemed to be readily available as far as I could tell, but my kids are too young for anything adventurous.
We booked our October 2017 cruise in February 2017, which is short notice by cruise standards, and late dining was the only available option. We are on the wait list for early dining, because my kids are used to eating dinner around 5:30 PM at home, so I think that would work better for us. Hopefully we'll get to switch.
Long winded answer, I know, but hopefully helpful.