What briefly lifts Victor's spirit on his journey home? Why is this significant?

The thought of his childhood friend and adopted sister Elizabeth cheers him up slightly. It is significant because he later marries her, and the monster kills her on their wedding night. The simple thought of returning home and the memories of his pre-monster experience.

Victor's spirits are briefly lifted when he makes a stop in the mountains near his native town and visits a beautiful lake. He finds some joy in the calm and peaceful sights of the lake, streams, and mountains. This is significant because later, in the dark, he views this scenery as threatening, a reminder of how natural things can be twisted and contorted into something scary (the monster made from just normal human body parts, those parts twisted into something horrific).