If your girls are a little bored with building camp gadgets and you want to challenge them to make something new why not consider a catapult. This can also be a fun bridging activity between Guides and Pathfinders or a competitive activity between two groups of Guides.
For a traditional catapult, try going to Glenn’s Scouting Pages for directions. This style of catapult is fired when one or two girls pull back on a rope, causing the swing arm to flip over.
Another popular style of catapult for lashing is a Viking catapult. There are several Instructables for building this style. In this style a small bucket (such as a large ice cream container) is mounted inside a triangle of lashed sticks on elastics. The bucket is pulled back and launched. Although this style is not what you might see represented in movies as a catapult, it is easier for the girls to adjust the angle and pull on the elastics, so it can lead to more experimentation to see how far you can fire your ammunition.
Finally, if you are planning to build catapults, give some thought to what you will fire and where. You want to make sure there is a large open field to fire in. Pick ammunition that won’t to serious damage if they hit someone or something. Water balloons or bean bags can be good choices. You may also want a target of some sort if you are having a competition. This can be a circle on the ground (who can get closest), a wall of blocks to knock over, etc.