I am currently a Multi-stake YSA Representative for my stake. My main responsibility is planning activities with the other YSA wards in the area.
Plan a variety of activities: service, cultural, spiritual, and sports. When you planning activities try to make them affordable. In addition, I would try to make sure you have the activity within an hour drive of most YSAs.
How many active YSAs are there in your stake and the neighboring one? That makes a difference in the type of activities you are trying to plan.
Service
Service projects in my experience work better with smaller groups like 30 or less. It will be less intimidating to a less active show up to an activity with a small group and easier for them to make friends.
Cultural
If you have a large amount of YSAs, then you can do a dance. I would suggest you have a theme that will excite people. A good themes allow people to dress up.
There are local festivals that happen all over the place. You can just have you and your friends meet up at the local ward building and car pool to the activity as a group.
Sports
My region has rented out a bowling alley for a night. We have done a basketball tournament. Volleyball has been popular with some of wards in my area.
Spiritual
Arrange a fireside on a Sunday evening.