You can try to group them with other things, especially physical things. Perhaps you have something you usually bring with you whenever you travel: a travelling toothbrush, passport etc. You can store those things all in the same place, and with them a note "if going to (city), try (restaurant)." When you start your trip, presumably you'll know whether you'll be going to that city, and can then set another reminder, such as a watch beeping at a specific time, or a piece of paper wrapped around all the money in your wallet or something.
If you usually plan trips a month in advance, you can have a reminder come up
in your system once a month, at which time you may know you're planning to go
to that city and can then set another, more specific reminder.
I wonder if http://hypothes.is will be useful for this sort of thing: perhaps (once that
system is functional) you'll be able to use it to set a comment to yourself which would
appear whenever you go to a travel-ticket-purchasing website.
If you have a map of the city you rarely go to, you could store a reminder with
the map, (a stick-note on the map), and hope you remember to bring the map with you!
If you have birthday candles, wrapping paper or such stored somewhere in your house, you can group all those things together, and also buy gifts ahead of time and store them in the same place too. Then when an occasion comes up, you can think "Do I have any gifts I already bought?" and go and look. You can label them so you know you bought them to give away! So you can go ahead and buy the funny present now; or put a note about it in with the other stuff.
You can use your memory. You can collect a number of things you want to memorize, and make yourself a quiz, e.g. flashcards. Question: what do I want to do if I ever go to (city)? Answer: go to (restaurant). The better you're doing at answering it, the less often you need to look at that flash card, so as you get more familiar with it you can move it to a pile you look at less often, so you don't get too bored with it. I don't consider this inconsistent with GTD: yes, you're using your mind, but it's a form of passive memory: you don't need to think about it except when the name of that city comes up. It's similar to learning a language or memorizing the capital cities of countries. It's also similar (in a different way) to the GTD system of looking at stuff during weekly review for the purpose of putting it into your mind.
Inability is an abstract thing involving comparison with alternate universes; it cannot be experienced.