Ice?

[A Guinea Pig]

Would it be a good idea to freeze a really big hunk of ice with hay in it? Would they like that? Or would that just be really bad idea since it’s too cold?

 

[Piggies&buns]

It would be a bad idea for many reasons - the hay would be soggy, piggies shouldn’t eat anything that is or has been frozen, it would defrost and make the bedding and consequently the piggies wet etc

If you are looking at ways to keep them cool, then this guide will help
Hot Weather Management, Heat Strokes and Fly Strike

 

[A Guinea Pig]

Sounds good!