Can a new, unlocked iPhone 5s be used via wi-fi before carrier activation?

I bought an unlocked iPhone 5s (32 GB, unlocked, T-Mobile SIM installed at factory) from the online Apple store for my wife as a Christmas present (our first smartphone). When she opens it on Christmas day, will she be able to use it out of the box using wi-fi, or is there something she needs to do first?

We have a 4th gen iPod touch and wi-fi, so she's familiar with the basics. I'm afraid T-Mobile and Apple Store will both be jammed up on Christmas and for days afterwards, so it'd be nice if she could just use it out of the box, without waiting to activate with T-Mobile.