Can you give some episodes that seem to have the same theme, plot or situation?

Beaver, the Model and Beaver’s Accordion

Both of these episodes have Beaver finding or receiving a ‘come on’ to get something just by signing. He listens to one Eddie Haskell and goes ahead and joins without reading the small print. Resulting in a considerable financial burden.

Any other episodes?

I'd say the one where he joins the record club. Sure, he had to pay some initial teaser fee but it was very small. As we know, he got in over his head really quick when a new 45 arrived every week, and had to be paid for full price, and as usual it all blew up in his face.

'Beaver's Library Book' maybe?

Another example of stupidity in small amounts over time finally escalating to a problem that near traumatizes him.

Sweatshirt Monsters and the Football Award episode.The gang wants to do something, Beaver goes thru with it, and the rest of the gang doesn't.

Ward's Baseball, Golf Club, and the Kite. Beaver doesn't want to play with something involving Ward, his enabler convinces him to, and the item gets wrecked.

Good ones. How about Garage Painters and Beaver and Ivanhoe. Ward has Beaver (Wally) read the classics like Tom Sawyer that are real boys’ book. He reads the book and tries to act out the scenes he reads resulting in trouble for everyone.

The burro, monkey, pigeons, parrot, dog at the beginning and later Poncho and the guard dog in the school hall, horse, Puff Puff and Bootsie Wootsie, baby alligator, Eddie's dog Wolf, fish for counting, the rat, the frogs, and anything I've forgotten--all problems with animals.