I grew up in a tiny branch in Mississippi, and how to hold effective Young Women's activities in a small unit is a subject near to my heart. I first began posting Young Women activity ideas on this site with the specific hope that smaller units with fewer leaders could get ideas on how to hold and plan activities without starting from scratch.
The new Church Handbook of Instructions has information on how to adapt the Young Women and Young Men's programs to smaller units:
Section 17.2.3: Young Men and Young Women
In a ward or branch with few young men, Aaronic Priesthood quorums may meet together for instruction and activities (see 8.11 and 8.13.1). However, individual quorums should still be organized, with leaders called and sustained for each quorum.
In a ward or branch with few young women, they may meet together for instruction and activities (see 10.6.2 and 10.8.1). A class presidency may be called for each age-group, or one presidency may be called for the combined age-groups until they are able to be organized into their respective classes.
If adult leadership is limited, the Young Men presidency may teach the Sunday lessons and administer the activity program without assistant advisers. The Young Women presidency may do the same for young women. In a very small unit, the Young Men and Young Women presidents may be the only adult leaders in their organizations. If this is the case, they teach the Sunday lessons and oversee activities for all young men or young women. When possible, counselors and a secretary should be called for each organization.
Because youth often benefit from socializing in larger groups, the young men and young women in two or more small wards or branches may meet occasionally for combined activities. If neighboring wards and branches have few young men or young women, the bishops and branch presidents may authorize the youth to meet together for weekly activities. When considering these options, leaders take into account factors such as distance and travel cost.
In a small stake or a district, the Young Men president may be the only stake or district Young Men leader. The same may be true for the stake or district Young Women president. When possible, counselors and a secretary should be called for each organization. (Church Handbook of Instructions 17.2.3: Uniformity and Adaptation: Young Women and Young Men)
17.2.4: Leadership Resources
When there are not enough qualified members to fill all leadership positions, the presiding officers may fill only those that are most essential. In addition to calling active, experienced members to leadership positions, presiding officers should seek inspiration to find other potential leaders among new members, inexperienced members, and less-active members who could return to activity. Church members do not need to be experienced or highly qualified to receive a calling. Having a calling is an important way for them to serve and to grow spiritually.
If there are not enough members to serve in leadership positions, presiding officers may organize their stake, ward, or branch on a smaller scale. Examples might include staffing some organizations with only a president and one counselor, having a counselor serve also as a secretary, and simplifying some programs.
In a very small branch that does not have a Young Women or Primary president, the Relief Society president may help parents organize instruction for the young women and children until Young Women and Primary presidents are called.
Leaders of very small branches follow the guidelines in the Basic Unit Program Guidebook. (Church Handbook of Instructions: Uniformity and Adaptation - Leadership Resources, 17.2.4
Here are some more ideas for holding Young Women in a Small ward or branch: