How about rave? From M-W,
Definition of rave
1 c : to talk with extreme enthusiasm • raved about its beauty
Using the given example,
Mary took John to the place where they sold the banana splits she'd been raving about for the past few days.
Though I believe rave suggests that she's already tried it and now she wants to show John that it's great.
A somewhat informal alternative is hype. From Dictionary.com,
verb (used with object), hyped, hyping.
2. to create interest in by flamboyant or dramatic methods; promote or publicize showily:
Using the example,
Mary took John to the place where they sold the banana splits she'd been hyping for the past few days.