Mystery Boxes are not a big mystery at all. A lot of confusion can be avoid by simply reading what the panels say. I know there are a lot of people racing through menus and pop-ups, but reading them DOES HELP!
One of the examples of mistakes is the Mystery Island goal "Go for a drive to scout the island"; about which the questions get posted like "How do I go for a dive?" DRIVE people, NOT Dive. When a goal mentions a specific item, use it, try out the actions there. If it doesn't mention an item, think about what is needed (examples are "Have a Pre-teen write a blog", "Play the Sims", "Watch Cat Videos", these can be found on the computer, as simple as that!)
Note: "Watch a 3 star TV" has a bug that does not recognising the use of a 3 star TV. Try the following: Have a Sim "watch the news" on a 3 star TV. Then, have a toddler tap the news program and "complain about the news." The goal completes! It doesn't give much, but at least you won't have to use LP's to skip. (Thanks Sue Dwyer).
The Mystery Boxes are simply a BONUS, nothing mandatory and nothing big. They are added to the normal goals. The normal goals have been expanded and made more random. (Three games tested with all three different goals).
You get a week to collect the following bonuses:
25% - § 1,000
50% - § 3,000
75% - 3 LPs
100% - Key and XP 5,000
Once you've finished the week's goals, the timer remains visible which shows when the next batch starts. You can still do normal goals, they just don't count for the week bonuses.
You get only ONE key per week!!!
A week lasts 7 days counting from the first day you received the Mystery Box goal!
You can either use the key or save them up for a bigger Mystery Box.
Contents:
You get the amount of items displayed in the panel, all from the Home-Store. They might contain items not yet available to you due to your level.
Standard prizes: ranging from 1,000 to 5,000 § or XP
Premium prizes: LPs, SPs, or resources
Common items: items costing §
Rare items: items costing LPs or very expensive items like the § 777,777 Pee Cleaner.
I have closed the comments for this post, if you are having trouble, please use the unofficial forum, there are many people there who can and likely will answer your questions: http://simsfp.perturbee.net
Technical support (if your game does not work as expected, like no goals, no building options, crashes and such) use: https://firemonkeys.zendesk.com/
This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License.
Based on a work at http://simfsfp.blogspot.com.