queues are always massive for the boats, it takes AGES. very good though. If queue is less than half way up round the enclosed area, join it. Otherwise it's too long.
Rollercoaster (jungle coaster) at back of the park is NOT worth queueing for, I've been on once on a quiet day, and it wasn't worth the queue then - on a busy day, forget it. The dragon coaster, at the far end of the park (the Castle) has very reasonable queues, and is a good ride. OTOH, Dragon's Apprentice is pretty rubbish.
The queues for the under-6 driving are not too bad, but for 6-and-over are massive. Probably could try the 6+ (he has to lie about his age), depending on how confident 5yr old would be driving.
Log flume (towards the right) is also excellent, Pirate Falls, but again the queues are MASSIVE. If the queue area is close to full, then you will be waiting over an hour. I believe it opens half an hour after everything else.
On a hot day, the water slide, Extreme Team Challenge, is fun, queues are generally ok. You shouldn't get too wet.
Don't queue for the Dino Safari - it's pathetic.
The Wave Surfer is quite fun, queues can be long, you do need to hold onto kids quite tight.
If all queues are long generally, try Miniland (lego models), Explorers Institute (walk-through, near the centre where the show is), Magic Show (same thing, walk-through) Duplo playground, Rat Trap playground (far side of park), waterworks play area near Duplo playground, Duplo Train, plus Fairy Tale Brook, which is a charming boat ride that ALWAYS has short queues, and is very pleasant