EXCLUSIVE

John Cena made the trek down to San Diego to try to win Nikki Bella back, but as we reported ... it may take more than just AAdvantage Miles.

Nikki and John were spotted together in the Mission Hills neighborhood of San Diego Saturday, strolling down the street after getting their fix of java.

John has been on a unrelenting campaign to win Nikki back ... after she broke up with him because he would not budge on the issue of having children.

Nikki told us a few days ago she now believes John has had a change of heart and would like for her to have his kids, but she says all the drama has made her question the relationship. She told us she loves John and would like things to work out, but she needs time to think.