About the design pattern, I recommend a DAO and Service approach, here's a good and short article about it: https://levelup.gitconnected.com/structuring-a-large-production-flask-application-7a0066a65447

About the models User/Activity relationship, I presume you have a mapped Activity model. If this is a One to One relationship, in the User model create a activityId field for the FK field and a activity relation where your ORM (I'll assume SQLAlchemy) will retrieve when user.activity is accessed.

class Activity(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    descrption = db.Column(db.String(255))

class User(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    activity = db.relationship(Activity)
    activityId = db.Column(db.Integer, db.ForeignKey(Activity.id))

If you have a Many to Many relationship you'll have to create a third class called ActivityUser to map the relation

class ActivityUser(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    activity = db.relationship(Activity, lazy=True)
    user = db.relationship(User, lazy=True)
    activityId = db.Column(db.ForeignKey(Activity))
    userId = db.Column(db.ForeignKey(User))

and you can retrieve the user activities like this (again assuming you use SQLAlchemy):

ActivityUser.query.filter(ActivityUser.userId == user.id).all()