If there is a water level outdoors, I think it would be cool to find some yellow shorts and a blue backpack Easter egg hidden in some bushes or hung on a branch(someone went skinny dipping?). Obvious to find or not.
Red feathers in a weird spot could also be a nod. Or a quest involving the collection of bear teeth and some red feathers.
Trowzer could mention a 2nd or third 3rd degree relationship with previous heroes or their trainers who've been through such and such training "Sossss I knownsss I thing or two, okayss?". Like he's establishing his credibility to teach new moves.
Transforming into a dryer instead of a washer by accident like in B-K
If there are instruments in a level's theme or a character's theme, there should be the appearance of a Banjo next to a Kazoo.
There should be a tiny key shaped ice in a drink, or in someone's drink.
If there's a nest which isn't a main feature but just part of the environment somewhere. It should have all the colors of the stop 'n' swop eggs.
That's al I got for now.