Apparently water buckets are a good topic if you look in the general forum 
In addition to the topic think of a way to structure the session to be productive. When I’m with a bunch of designers I don’t really need to set up structure because they are more comfortable throwing ideas out there. Sometimes with non designers I’ll set up lighting fast exercises. Below is an example session flow:
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Ice breaker. Everyone introduce themselves and answer a particular question (since we are a speaker company I usually ask people to name the first album they ever bought with their own money, or first concert). List out on post it notes and post.
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research grounding: 10 min presentation on the topic
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assumptions, everyone gets 5 minutes to write down as many assumptions about the topic’s parameters. Write each on a separate post it.
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10 min share out, each person shares their assumptions, moderator sorts them into themes as they go. 5 min share out.
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random entry. split into groups. Each group gets a random object and has to list as many descriptors to that object (i.e. if object was a spoon you would get things like, smooth, simple, functional, metal, silver, handheld…)
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each group gets a shuffled assortment of assumptions and descriptors. As quickly as possible they have to generate as many solutions as they can that both ignore one assumption and use 2 descriptors. 15 mins. Write on an 8.5x11 piece of paper. Give the idea a title, wrote a brief description, and then a small doodle to visualize.
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team share out, 15 mins. Moderator posts on wall in a grid as they go.
moderator posts on the wall a giant 2x2 with tape (one axis might be feasibility, the other might be cost, or desirability). Moderator leads group in an exercise to rank on a scale from 1-10 on each axis and then post on the 2x2 in the appropriate place.
This usually leads to some pretty out of the box ideas. It helps if there is a designer in each group pushing people.