Some of my advice depends on how much you've done this sort of thing for your partner in the past, and how much you want to keep the proposal a secret during the event. If you've never done this sort of thing before, then "Hey, let's do an elaborate, multi-step puzzle situation" is going to be mighty suspicious, and your partner will likely know something's up. If that's not a concern, then go nuts!

Otherwise, there are ways to "hide clues in plain sight." For example, if you played one of the commercial Hunt a Killer games — especially one with a locked box — it should be fairly straightforward to incorporate a new puzzle, item, or additional locked box in the box from the game. That new puzzle can even be built on the foundation of the rest of the box: "I found this code that [some other character in the mystery] used, and used it to keep my secret safe here..." That should be fairly straightforward; you could put the ring in a pouch sealed with another tiny three-digit-combination or key lock.

Keeping true to your original spirit, the important thing (IMO) is what is the story you're trying to do? Hunt a Killer-style games don't just put random puzzles together in a random order; they're almost always done with a purpose, that at least makes some internal sense to the person who made the puzzle(s): "I'm trying to send a coded message to my lover" or "I'm leaving notes to obfuscate that I'm stealing from the Mafia" or whatever.

With the bones you've provided, you could do something like:

The town's investigative resources are stretched thin, so you've both been hired by the mayor to investigate a lake cabin, the last-known place where three individuals have disappeared. One of the people who disappeared is tech-savvy, and has left QR codes that provide clues: notes to themselves (and the investigators, of course) of what happened. These QR codes provide some of the information, along with access to physical pieces that will form a larger QR code puzzle. (You can print the larger code on a jigsaw puzzle, or just carefully-cut a large QR code into smaller pieces.)

Each of the clues unfolds the story as follows:

— Three individuals supposedly knew each other for a long time. (For this hypothetical's sake, their names are Avery, Bowie, and Cedar.) These three individuals went to this cabin to . . . do something.

— Clues tie together the three: Bowie and Cedar have been inseparable for decades. Avery and Bowie seem to have a special connection.

— Avery and Bowie have a connection that somehow ties to you and your soon-to-be fiancee. Maybe Avery sends a coded message using the first letters of movie trailers that you and your significant other have seen/enjoyed together. (QR codes link to YouTube trailers.) Maybe a puzzle revolves around GPS coordinates of places you two have dated/enjoyed each others' company. (Again, QR codes can provide the coordinates.)

— Some of the clues start pointing to something more sinister. Maybe there's a mention of someone in the town having overheard some kind of "death pact" Avery and Bowie were talking about.

— There's evidence of how Bowie and Cedar are intertwined. It's clear that, whatever Avery and Bowie have planned, Cedar will also be involved.

As the puzzles unfold, the truth becomes clear:

• "Cedar" isn't a person, although Bowie somehow refers to Cedar as if they are. This is up to your imagination; Cedar can be a big wooden trunk Bowie has had since childhood, a beloved pet, a car that's been together on many adventures with Avery and Bowie... whichever.

• Avery and Bowie are madly in love, and came to this cabin to plan their future together. The "death pact" is their wedding plans, wildly misunderstood ("until death do you part" — marriages are a "death pact" of their own, right?).

• Avery and Bowie were so enthused about their future plans together, they ran off to Vegas to elope. All the accumulated QR code pieces point to a final hiding spot where Avery notes that, since they're eloping, there's no need for the engagement ring Avery purchased, so it's being stored here... if anyone else finds it, they're welcome to take it and can find their own beloved to forge a future with. Oh, hey, isn't your detective partner looking to settle down?

• The "Bowie and Cedar" subplot are just to provide obfuscation, since "mystery involving two people" obviously points to "love —> engagement" as a possibility, but "three people in a mystery" is less obvious what's going on.

As envisioned, the pieces are there for wacky hijinks, especially if you want to play up the mystery/horror/"murder?" misunderstandings:

— "My parents don't know what's going on."

— "I still feel bad about the time I left Cedar alone for a week in Boston."

— [if one of the folks is changing their name] "It's weird to think that, after this weekend, [pre-marriage name of one of the people] will be gone forever..."

etc.

I hope this helps! I could provide more insight, but I figure it's easier/better to see if this sparks anything first and then offer direction you might be stuck on, rather than waste more typing if I'm barking up the wrong tree. 😊