I'm planning to get messy sometime soon and am looking for ideas of foods to use and would love to hear what others like to get messy with

Custard is a staple for any splosher. But anything sweet and smooth for me

Yeah, custard is a great place to start. So is cake mix!

Angel delight, with custard and whipped cream on top!

Can I add to the question, I'm going to get messy soon too but have to do it outside, what biodegradable mess is there I can use outdoors? I suspect none but someone might have a smart idea?

Cake mix, just add water, nothing else. Also some big box stores sell giant cans of pudding. If you want to keep things warm, put closed containers in a warm bath (I've used a hot bath - heat dissipates fast). Chocolate syrup is fun.

Straight up bakery goods - birthday cakes, sheet cakes, real pies.

Well porridge is awesome as well as custard and peanut butter and chocolate spread

Definitely pudding. Especially in the big cans....you can get chocolate, vanilla, etc. easy cleanup and awesome to use.

These are my favourite things to get messy with too, but add in Treacle/Syrup, Marshmallow fluff, and of course Custard!!

My favorite is chocolate syrup.