Baby-Joel doesn't need CGI; it's all a visual trick called forced perspective. We infer the size-distance relationship using visual cues and previous experience, but Gondry is messing with our assumptions. (Source.)

Improvisation was the name of the game in Eternal Sunshine. The elephant parade scene, for example, was a complete coincidence. Clem punching Joel goodbye and even the scene where Stan and Patrick break out into laughter instead of reciting their lines were also improvised moments. (Source.)

One of Gondry's favorite shots is a quick view of Joel looking for Clem at the elephant parade. In the movie, Clem has been erased from this happy memory, but in reality Gondry pulled Kate Winslet away from Jim Carrey, and Jim was genuinely concerned and looking for his co-star. (Source.)

In the original screenplay, we get to see an old Clementine going to Lacuna and erasing an old Joel from her memory. It's obvious that the couple has been erasing each other from their own memories every few years for decades, never able to break the cycle. (Source.)