Submarine:
https://www.youtube.com/watch?v=MF8zk0jmAvE&ab_channel=YuriOstr

safety: 5 very safe

difficulty: 4 probably a bit hard to control the outcome and might be hard to make. if i make it and realize something is wrong and it doesn’t work, its very likely that i probably will be completely out of time to even try to do it again, but it doesn’t include motors or anything so i wont have to worry about that

energy transfer: elastic potential (rubber band) ==> kinetic energy (spin propeller)

Car Boat thing:

https://www.youtube.com/watch?v=izKJVYAbgso&ab_channel=7dayscraft

https://www.youtube.com/watch?v=BnzxebhBEEs&ab_channel=DiyPortal

safety: 5 very safe

difficulty: 3 the propeller and the wheels use basically the same spinning mechanism and uses the same motors, so right now it feels like that all i have to do is to add propellers to the wheel somehow (and make a body suitable for water/that can float)

energy transfer: chemical potential (battery) ==> electrical energy ==> kinetic energy (spin propeller and wheel)

flying butterfly thing

https://www.youtube.com/watch?v=jVmPVtzYeAQ&ab_channel=【BenriLifehack】便利ライフハック

safety: 4 it might hit someone but its not very hard

difficulty: 2 its probably way too easy but it looks fun

energy transfer: elastic potential (rubber band) ==> kinetic energy (potential energy is released and spins the back of the butterfly thing)

helicopter or plane

https://www.youtube.com/watch?v=RgVaid5HnB8&ab_channel=S-DiY

https://www.youtube.com/watch?v=i8NpJzu8Lek&t=310s&ab_channel=THANGEngineer

safety: 4 it might hit someone but wont do much harm

difficulty: 5 probably very hard to make and a lot of things found on youtube could barely fly, its also quite hard to control where it goes

energy transfer: chemical potential (battery) ==> electrical energy ==> kinetic energy (spin propeller)