Nathanael just witnessed evidence of Jesus being legit that got him -- even with initial doubt! Pretty cool!
From Nathanael's change in perspective, Jesus invites Nathanael to believe in him (1:50). However, most importantly, Jesus reminds Nathanael (and to an extent, the other disciples listening to Jesus), that he isn't done. Jesus promises that they will "see greater things than these". (1:50) There's a lot for them to look forward to, and many of them are described in later passages.
Among the "greater things" include "heaven open[ing]" and "angels of God ascending and descending" (1:51). That sounds a lot like stuff you don't normally see on a daily life. Later on, Jesus would share them all that -- and their significance.
And we reached the end of John 1! John sets the stage of Jesus (as God), and after some preparations, John (and his disciples) begin their journey of following the Son of God.