I've organised a few weight loss challenges over the years.

A really "fair" way of doing it - is % loss.... like they do on Biggest loser.

Then someone who is bigger (thus more to lose) - is on even ground with someone who has 2-3kgs to lose.

I can give you some spreadsheets & a website link that I set up with loads of motivational articles... just pm or email me if you would like that info.

I think that you should set weekly goals. things like: drink 6-8 glasses of water each day. take min 3 walks... etc. Some people do that anyway... but some people (like me) don't do it at all. And to set it as a challenge - hopefully it becomes a habit.