I'm an old school kind of parent in many ways.
Just because the kids are bored doesn't mean they need to have stuff planned. I often tell my daughter to use her imagination. Unfortunately she's an only child, so there's no one around to play with. She often ask to have a friend over or if she can go there, and of course I allow that. Well, most of the time anyway. If I can't be arsed, then I'm saying no.

Her friends love being here, they think I'm cool. I think it's because I'm pretty laid back and let the kids be kids. I tell them to be careful if they start doing something that most consider 'dangerous', but I let them do it. Sometimes they hurt themselves, and when the storm has settled, I tell them that can happen when they do whatever they did. Part of their learning, IMO.

Being bored is also healthy sometimes. If nothing else, you can try guiding them through a relaxation.
Tell them to lay down, hands on their stomach, and feel their hands rising as they take a breath in and how they fall when they exhale.
Then go on to feel what parts of the body touch the surface and what parts doesn't. Is there a different feeling on the places that touch? Etc etc.

There are countless possibilities.
IMO, the worst thing to do is to give in and plan their day for them.