The new Cub Scout delivery method encourages holding two regular den meetings a month and using the extra week for an activity or outing. One possibility, especially in the colder winter months, is to do a service project with the boys. It can be difficult, though, coming up with service project ideas for this age range. Here are some websites with suggestions:
366 Community Service Ideas from UNL
Community Service Ideas For Kids from Buzzle
Developing Service Project Ideas For Younger Children from Corporation for National and Community Service
Community Service Ideas For Kids Of All Ages from Kid Activities
LDS Service Ideas from About.com
Service Projects For Kids from suite101.com
Pocket Flag Project
Boys this age can learn to tie quilts (a good reinforcement of square knot tying); read to younger children or the elderly; draw pictures and write letters (for troops, hospital children, rest home residents or missionaries); sign up to clean the church as a unit when it's your ward's turn (a good activity for involving families too); clean up trash in a neighborhood, park or canyon. If your area has a festival of trees you can plan ahead for next year. After Christmas artificial trees can often be found on clearance for a very low price. If someone in your pack is willing to get a tree and store it for a year, the boys would have a great time making ornaments and decorating a tree to donate next year. In the summer or spring an environmental service project can be tied in to the Leave No Trace Award. You can find suggestions for projects to meet the requirements for the award here. You'll need to contact the groundskeepers of local campgrounds and hiking trails to find out exactly what is needed.
You may need to call around to various organizations in your community to find out exactly what it is that they need that the boys can do. Some may prefer visits, while other may ask for drawings and letters or quilts. Try contacting your local senior center, hospital, museum, animal shelter, homeless shelter, foster parent association, schools or library.