A party bag, or a "goody bag" as it is often called, is a way to show your party guests your appreciation for attending your event. They don't have to be filled with outrageously expensive favours, they just have to be thoughtful and appropriate for the occasion. There is no rule for what to put in a party bag or what kind of packaging you should use, just make sure that you have enough for all of your guests.
Relate your party bag to the occasion. If you are throwing a child's birthday party, you should always include candies, small toys and other mementos like whistles, party blowers and even streamers for the kids. If you are throwing a baby shower, novelty baby items and decorations are appropriate.
Decide on the type of packaging. There is no science to this, nor is there any ruling that you have to purchase bags from a store. You can choose to make bags out of inexpensive material to customise or coordinate with your event. Or, you can purchase inexpensive favour boxes from your local craft store that come pre-decorated. The choice is yours and depends on what you intend to put inside the boxes.
Make all of your guests feel included. Assess what type of people are coming to your party so you can choose gifts for inside the party bags that they will appreciate. If mostly women are intending your party, body lotions and fragrances are a crowd pleaser. If it is a mixture of men and women, choose gender-neutral items like books or gift tokens. For kids, candies and toys are always a safe option and appropriate for any gender.
Think of it as a way to memorialise the day. There is no obligation to offer party bags, but they are always appreciated for guests who took time out of their day to celebrate you. They are especially necessary to anyone who helped you plan the event. Therefore, make sure the party bags say something about the event. For instance, baby shower party bags can have baby-themed decorations or mementos. Birthday parties can include pictures of the celebrator. The options are endless depending on the type of event that is being celebrated.
Personalise it. Nothing says appreciation more than personalised gifts. Include candies or mint tins that acknowledge the day's event on the label, complete with the name of the event and the date. Or you can choose to offer monogrammed gifts that your guests will appreciate.
Take a count of your number of guests. The biggest party thrower faux pas is to not have enough party favours for everyone. It is a good idea to make extra, just in case someone at your party brings along a guest. You might spend a little extra, but you can feel comfortable knowing that everyone will receive a party bag.