We are flying from NC to Spain on Christmas Day arriving in advance of a cruise leaving Barcelona after staying there in port for a night. My questions are: we have four nights on our own before boarding and there will be a day excursion in Barcelona after boarding. Would there be enough things to see & do for five days in Barcelona or would a flight into Balencia for a couple days make sense, taking the high speed train from there to Barcelona? We have been to many European countries but not Spain & we are looking forward to visiting during a festive, cooler & hopefully less crowded time of year.
How much time you need in Barcelona depends on the sites you want to visit. There are day trips you could take from it if you felt you’d seen everything on your list. Valencia (assuming that’s what you meant) is certainly an option for a couple nights.
Barcelona has way more than enough things to do - on its own - to fill five days.
The upside of staying and focusing on Barcelona is it is a great city, it simplifies logistics, and you can cover quite a bit of the "must see" items there in five days at a reasonable pace.
The downside of staying solely in Barcelona is that Barcelona is definitely just a tiny slice of "Spain", so your experiences there will not reflect the rest of Spain (like visiting NYC alone). That's also an "upside" as it gives you a great reason to return later for more explorations in Spain beyond Barcelona.
If wanting a day or two beyond Barcelona, there are always the simple day trips to places like Montserrat, Girona, Stiges, etc., that are manageable and low on logistics complexity.