Not as tidy, but I have a hobby of collecting Starbucks mugs from cities around the world. Leftover currency and transit cards end up in the mugs in case I need them for a subsequent trip. I'm up to about 75 mugs around the house at this point. Any small trinkets/souvenirs (basically limiting myself to things that fit in the mug) also are stored there. The only challenge is remembering that Euros always go in the Paris mug (not sure how it got designated) and leftover currency from countries I have yet to get the mug end up in adjacent mugs (i.e., Uruguayan Pesos in the Buenos Aires mug).

When I was much younger and going to be a bit too far from home for comfort or be in some unknown place, I used to always try to run around with a currency note in my shoe and/or sock, and it usually would be something equivalent to 5-20 hours of the wage for the average local McDonald's worker. Same thing if I went away to camp. While I no longer do that, the habit of wanting cash along with me most times has obviously not ended based on what I'm carrying with me today.

I am walking around with a collection of the following valid currency notes: USD, EUR, GBP, CAD, SFR, NOK, DKK, SEK, MXN, INR, ARS, BRL, CNY, THB, NZD, SGD.
I am walking around with a collection of the coins for the following: USD, EUR, GBP, SFR, NOK, DKK, SEK and CAD.

I also have some Belgian Franc, NOK and SEK notes that have been sort of demonetized. For those, I have to get the central banks to exchange them for spendable currency, but I'm holding off on doing that exchange with the old SEK notes until my collection of them is far more substantial.

Everything else I try to get rid of and run down before I leave the country of issuance. Surprised I have no AUD and JPY left.

Just so you can say you are a trillionaire!

I've never liked having a large value of cash "stranded" in a foreign currency. Even when I know I'll return to a country within a few months I prefer not to hold more than about $20US worth of its money. This is because 1) plans change-- that next trip may not happen, 2) it's a PITA to change foreign currencies back in the US without getting raked, and 3) it's rarely hard to get cash upon arrival. I've been to very few places so far where I haven't been able to locate a fee-free or low-fee ATM at/near the airport with a bit of advanced research.

The one time I made an exception to taking a lot of foreign money home with me was when the Korean Won crashed relative to the USD, trading 40% lower than the previous few years. I knew I'd be back to Korea at least a few times within a year from that point so I withdrew enough for 3 trips worth of normal out-of-pocket cash expenses.

I don't keep large amounts either. While I will keep USD and GBP worth under CAD$100 larger amounts get exchanged back to home currency. I'm fortunate to have a currency exchange outlet not far from home with reasonable rates.

I don't see the need to keep much. I have a drawer with about 50 euros, 20 pounds, and 30-40 Canadian dollars in it.

I typically try to keep at least one of each denomination of coin from a country that I visit for the first time. A sort of souvenir, if you will. Otherwise I try to spend as much of the cash as possible before leaving.

The currencies we intentionally hang on to are GBP, INR, JPY, EUR and MXN, all of which get used on a regular basis. I can't recall the last time I had to use cash in Canada.