A wonderful way to engage your guests is to plan weekly activities. If your hostel can support hosting these activities, it is a good idea to set up a weekly schedule that lists what is happening when. You can choose to keep these activities on the same days and times each week or rotate them around to keep things from becoming boring. You can even have some activities one week and others the next.
If you cannot do the activities on your own, you may want to consider promoting weekly happenings in your area. This is a great way to support the other local businesses in your area and they will recognize that and keep you in mind when someone asks them about a great hostel in the area.
A few activities that you can include are comedy nights, free tour days, museum days, open mic nights, cheap movie nights, and morning or afternoon hikes. Of course, there are many other options available to you and your guests may even have some amazing ideas that they can share with you.