A cat or dog would be most likely, but I've thought for years that a hedgehog as a pet would be fun, so we'll go with that.

The next poster is guaranteed a bumper crop of any one fruit or vegetable this summer.

Oh that fruit would definitely be strawberries..... I'd have tons of half flats to share.... and tons of half flats to myself.... and of course, freeze a ton so I have strawberries the rest of the year, but you can't beat fresh strawberries, so you bet I'd be eating it every day, as well as making them into smoothies, preserves, baking them into muffins, etc.

The next poster is about to pay for their food, but someone else fits the bill for you.

Louis XIII (pizza $12,000) 

The next poster would find 300,000,000 million dolls that are alive and you either live with them and get millions of dollors or don't live with them but they will stay there. 

I would leave them there, because I'm scared of dolls.

The next poster is guaranteed to become fluent in any new language they want.  

I'd choose Welsh, as I live only about an hour's drive from the border of Wales (although it takes about another hour to get to the parts where there are lots of Welsh speakers)!! I am already learning the language, but haven't had much time to devote to it lately with the many other things that are going on in my life at the moment...

The next poster gains the ability to shapeshift at will into any one animal of their choice (can be mammal, bird, reptile, fish, insect or whatever you like!).

I think I would transform into a large bird, maybe a bird of prey, so I could fly and see things from a bird's eye view.

The next poster has the opportunity to adapt any one obscure work (AKA not a CoN or other wildly popular work) into a movie or mini-series.

I would turn my diary into a live-action movie. 😏 

The next poster is given a time machine and an umbrella.

Sounds like the perfect situation to always know if it's going to be raining on any important days!

The next poster is given an unending box of chocolates.

As someone whose wish has often been for an unending box of chocolates, I would be in high heaven. Though likely in need of some self control . 

The next poster can transform themselves into any character from a book and live in their shoes for a year.