If you'd like to read about a potential history model, you might check into Eleanor of Brittany, the "Fair Maid." She was a granddaughter of Henry II and Eleanor of Aquitaine (daughter of a younger son, so in line for the throne though not a straight-up princess, but potentially close enough). What makes her work so well as a model is the amount we know about her life thanks to a rather unfortunate circumstance. She actually spent most of her life as a hostage at a couple other courts, whose nobles went to significant lengths to document her high-quality living circumstances (proof she was alive and treated well) and the security it took to hold her.

As far as religion goes, daily Mass is probably a stretch for a non-nun. It is very true that strong piety was an topos of medieval noblewomen, although the degree to which that reflected reality surely depended on individual situations. An early 13C noblewoman like Eleanor might well have owned a prayer book, which would be in Latin. She probably would have prayed out of that on her own. However, we think that a lot of "reading" in the sense of romance and epic literature (Arthur etc--Eleanor lived during the composition of the so-called Vulgate cycle of the Arthurian legend) would have been in a group environment with one person reading out loud. This would have been in the vernacular.

Distributing stuff for the poor is...interesting. It's kind of a hagiographical topos in the High Middle Ages (11-13C) that women saints, as adolescents, are always trying to sneak food out of the castle or their father's house to the poor and get in trouble for it. Clothes for the poor...I'm not familiar with any cases of that, although you could go for it. More likely, charity would be prayer, donations to a chantry priest to say prayers for the souls of her family and country, and perhaps physical asceticism. Sewing would more likely be as favors for her father's/husband's favored courtiers and their wives, or for her mother and her mother's ladies in waiting.

Depending on how you want to play with time frame, other important pastimes would have been enjoying a pet dog, chess, riding (we know Eleanor did a lot of this; she is somewhat frequently gifted reins and gilded saddles), perhaps traveling to other courts (or the court itself may have moved from place to place). Music and dancing for sure. Keeping up with gossip (that's a real thing--it was how relationships and reputations were made, showing who was an insider versus outsider). Some princesses--I am thinking here of a later case, Kunigunde of Austria who became Duchess of Bavaria in the late 15C--were quite active as intercessors between supplicants and their fathers. You may also be familiar with the practice of sending noble children and adolescents to be fostered at another court, to learn social customs and expand their "social networks."

Basically, I guess, you have to decide whether you are going for "generic medieval" where you mash-up bits from different eras to suit your story, or shoot for historical accuracy by focusing in on a specific case. I'd suggest Eleanor as a model if you want a specific case, or you could go for early Tudor England where we have, again, good insight into daily lives of women at court. There's some great stuff on Kunigunde out there but it is, IIRC, all in German.