The following short paragraph was taken from Virginia Woolf's Mrs Dalloway:

Mrs. Dalloway said she would buy the flowers herself.

For Lucy had her work cut out for her. The doors would be takenoff their hinges; Rumpelmayer's men were
coming. And then, thoughtClarissa Dalloway, what a morning--fresh as if issued to childrenon a beach.

What a lark! What a plunge!

What does "what a lark! What a plunge" mean? I consulted OED, but "lark" and "plunge" are both polysemies … and i've also Googled the question, but there's no helpful infomation. I need your help.
These definitions seem to fit the exuberent context:

a merry, carefree adventure; frolic; escapade.
a headlong rush; splurge
Yes, Mrs D is looking forward to this fine day.
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guzhao67What a lark! What a plunge!
This has got to be one of my favorite passages in all of English literature. Emotion: smile

thank you