How about rave ?
From M-W, Definition of rave raved; raving intransitive verb 1 c : to talk with extreme enthusiasm &bullet; 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, hype 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.
ell.stackexchange.com/users/36187/Em.
gushɡʌʃ/verb 2.
Speak or write effusively or with exaggerated enthusiasm.
Mary took John to the place where they sold the banana splits
she'd
been gushing over for the past few days.
ell.stackexchange.com/users/40227/Sam
The first word that sprang to mind, on seeing the title, was jabber verb
[NO OBJECT] Talk in a rapid, excited, and often incomprehensible way.
‘he
jabbered on about football’ noun Rapid, excited, and often incomprehensible speech.
Source https://en.oxforddictionaries.com/definition/jabber
ell.stackexchange.com/users/6477/Mawg says reinstate Monica
How about enthuse
Express eager enjoyment, interest, or approval regarding something. https://en.oxforddictionaries.com/definition/enthuse
Example: Mary took John to the place where they sold the banana splits she'd been enthusing about for the past few days
ell.stackexchange.com/users/21275/GeoffAtkins
You can say ".........she'd been raving about.......".
Enthusing about is also possible.
ell.stackexchange.com/users/9259/Khan
My first thought was blather .
to talk foolishly at length —often used with on
So Mary took John to the place where they sold the banana splits she'd been blathering on about for the past few days.
ell.stackexchange.com/users/51258/user51258
To bubble.
Mary took John to the place where they sold the banana splits she'd been bubbling about for the past few days.
ell.stackexchange.com/users/51288/Jonathan
Extolling?
To bless, carol, celebrate, emblazon, exalt, glorify, hymn, laud, magnify, praise, resound
Does change depending on how Mary feels about her own enthusiasm.
Jabbering, going on about, blithering etc implies she's aware
it's annoying.
Or more something that John might say about her mentioning it so much.
Raving is more neutral or enthusiastic.
And something like extolling feels quite character dependent, if she's that sort of person...
ell.stackexchange.com/users/110441/user110441
Is there a word for "to talk excitedly about something"? I'm sure there's a word for that. But  Thesaurus.com  didn't give me the answer.  Example sentence:   Mary took John to the place where they sold the banana splits she'd  been ______ for the past few days.
Word to describe "to talk excitedly about something"?
ell.stackexchange.com
<word-request>
ell.stackexchange.com/questions/122737
ell.stackexchange.com/users/43585/alex
To hype, gush, to blather, to rave, to enthuse, to extol and to bubble are all suggestions in this context. Some words that could be used in this context are: "rave", "hype", "jabber", "enthuse", "blather", "bubble", "extoll".
Some words that could be used in this context are: "rave", "hype", "jabber", "enthuse", "blather", "bubble", "extoll".