DS1 is 5 (almost 6) and is complaining a lot about being bored. I can see that partly he's clocked that it winds me up a bit, but also he seems to be at a bit of an in-between stage where he is genuinely at a loss at what to do.
He used to happily play with trains / cars / transformers. He seems to have outgrown this, but not really grown into anything. He likes Lego, but doesn't want to do it all the time. When we go to the local park there's not any climbing frames or things that he likes, but he also doesn't like football (which most children his age that go seem to do). He's never been a child interested in crafts, or sitting at the table colouring in. He loves being read to, but his reading isn't good enough to read for pleasure himself yet.
He's incredibly sociable, and what he really wants to do is meet up and play with his friends all the time. In the absence of this his main solution is to wind up his little brother. I'm at a bit of a loss of what to do with him! It was easier when I just had to pretend to be a station announcer.