Doing something really hard, challenging or just outrageous can raise a lot of money. It will also raise media attention and bring publicity for your cause which can be an added bonus.
Some things to bear in mind:-
1 carry out a risk assessment and make sure it is reasonably risk free. Do not let anyone attempt anything that they are not physically prepared for.
2 Make sure you let the press and media know well in advance. There is no point in doing it if you miss out on that all important photograph.
3 Get the sponsors first. This might be the harder task than the challenge itself.