I was with you until you started saying the "they have to keep it secret" or whatever nonsense, just make it an actual shopping task with no bloody "hidden task" element
Something similar but a dictatorship task where one HM is given the role of dictator (either by the public or by other HMs),
gets to wear a fancy general's outfit, gets to overrule a nominee... they have to assign other housemates labour tasks (think the BB6 Big Brother one with sorting out colours of maggots), create and perform a new national anthem, and have to do military marches around the house any time Big Brother plays some fanfare (I know this has been done a lot but it's an easy way to piss them off)
So it'd be split into various parts that all have to be passed (or only "3 fails" like Laura doesn't take part in the anthem or w/e):
- The national anthem performance
- The labour work
- The marches.
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Another idea: a TV station task. I'm not sure if this has been done but has probably had some elements of it done before. Housemates are split into groups that have to work on a separate TV show together.
- One has to produce a chat show. With one or two HMs as the presenters, and some special guests enter the house to be interviewed (knowing C5 these would be ex partners but I'm all up for them being total oddballs like the country's oldest morris dancer or whatever). Has to last half an hour and one of the housemates has to be talking at all times (or incur a fail).
- One has to produce a kids show. Three or four housemates are given costumes/makeup to teach the viewers basic numeracy and literacy, but also keep on happy facades. Think the Drag Race s5 challenge:
https://www.youtube.com/watch?v=Cqi50U04lBo . Could make this program longer to drive them crazy I guess with the happy-happy-house vibe. Perhaps give Georgina a role
- One has to produce a cooking programme. If it were possible then to devise some kind of malfunctioning kitchen task room would be great (sort of like the McDonalds advert recently with the guy on the speed bumps) - two or more housemates have to follow a recipe from scratch whilst talking to the camera and acting professional, but are plagued by problems like fans blowing flour everywhere. DO NOT BLOODY FEATURE I'M A CELEB-STYLE 'GROSS FOOD' CHALLENGE ELEMENT.
- And the fourth group have to act out a couple of soap opera scenes that they can create themselves. I trust them to have some natural comedy