The first chapter where Bilbo actually starts to show some kind of justification for Gandalf's confidence in him. Not coincidentally, it is the first chapter where he is on his own, and there is a point when he decides to go forward by simple elimination of all of the other possibilities. It's a simplistic sounding method, but in real life it is often surprisingly effective to eliminate all of the impossible or implausible options, and realize that you have only one left. Otherwise, you can end up spinning and spinning in your head, looking for an option that has no apparent downsides, and that may not exist. Certainly, in this case, Bilbo's least bad choice is still not a good option (hurtling forward down a dark passageway without any clear idea of where he is going), but he makes the best of it. His capacity for not giving up in despair or terror is demonstrated here, as he is far from not only any help or support, but also far from anything he has any knowledge of.
One thing about the Riddle Game which Gandalf commented on later, in LotR, was how both Bilbo and Gollum knew many of the same riddles. The mention of Gollum's grandmother and his distant past aboveground, under the sun, are also intriguing. Even in the original published version of The Hobbit, when JRRT knew nothing yet of what the ring was, he hinted at a backstory for Gollum, in a place and lifestyle very different from the one he now had. Presumably he was just wanting to make it a more intriguing character than it would have been if it had spent its entire life in a dark underground lake? Or just needed to explain how it could answer riddles from an above-ground Hobbit?
It is interesting, when reading about the debate as to whether or not the dwarves should go back and attempt to rescue Bilbo, to consider that no one (not even Bilbo or Gandalf) considers for a moment going back and attempting to rescue their ponies. It will be a good illustration of why, in a later chapter, Beorn does not let his ponies go very far with this group. Ponies, on the whole, do not fare well in this story.
Chapter 6