Hi Nicola,
Thanks for bringing another question to us here at planDisney! I'm happy to assist. 
You are in for a most magical visit next August and I'm sure you must be excited to take advantage of the Dining & Merchandise Credit associated with your 
2024 Early Booker Package. This is such a great bonus to an already magical vacation so let's be sure you're ready to make the most of it when you arrive.
You'll receive your credit via email to the address associated with your reservation on your arrival day and can begin savoring right then and there, from 
Character Dining to shopping at 
Disney Springs. Though guests can 
redeem this credit at a number of dining and merchandise locations in the theme parks and resorts, unfortunately, you won't be able to use it to purchase 
theme park tickets or 
Disney Gift Cards, as indicated in the fine print on page 5. I know this might not be the answer you hoped for, but there are plenty of delicious dining experiences and shopping fun awaiting you next summer! 
Since you're interested in 
Mickey's Not-So-Scary Halloween Party I'd recommend keeping an eye on 
Disney Parks Blog for all the details on the 2024 dates as they become available. This year resort guests were able to purchase their party tickets beginning in late April, so you might not want to wait until your arrival to select a party date as these popular events can and often do sell out.
Nicola, if you have additional questions please don't hesitate to come back and ask us. We're always happy to help!
Magic and memories,
Amy