Is there any better way of tagging a time capsule other than
tourism=attraction
?
It’s debatable whether it’s a tourist attraction at all. “Oh look, it says there’s a time capsule there. Let’s go visit it and gawk at a slab of paving stone that has the words ‘Time Capsule’ chiselled on it. I can photograph you standing on it. Much better than the Eiffel Tower.”
In fact, is there any point even tagging it? It exists. If you’re a tourist and nearby then you might make a small detour to take a look. But it’s not that big a deal.
Thoughts, anyone?
There are 2 “historic=time_capsule” in Disneyland - https://taginfo.openstreetmap.org/tags/historic=time_capsule . There are 10 other things named “Time Capsule” - https://taginfo.openstreetmap.org/tags/name=Time%20Capsule . Maybe some of those have tagging that’s worth copying from? Again, “historic” seems to be the preferred main tag there.
I suppose historic=time_capsule is a possibility, although it doesn’t seem to be a sanctioned attribute of historic, according to the wiki.
I had a quick look at one of the sites you suggested, and it seems to be used there for what I want to do.
Certainly, “historic” is a little better than “tourism.” Nobody is going to plan their holidays around seeing a time capsule. Not unless they have the opportunity to open it or see it opened. But…
The problem with using “historic” for a time capsule is that it’s a very small value of “historic.” At the extreme view, everything on the map is historic. The preceding sentence is historic.
But unless anyone comes up with anything better, I’ll probably go with historic. I’ll leave things to fester for a while, just in case somebody adds the tag “crappy_thing_you_might_want_to_look_at_but_wouldn’t_plan_your_holiday_around.” 
I wouldn’t worry too much about what the wiki says, especially for things that are rare in the real world. It’s pretty much impossible for someone writing a wiki page to think of all of the possibilities that might fit under a certain tag.
Yeah, I understand the difficulties of anticipating everything. It’s why things keep evolving. And, like biological evolution, we get vestigial appendages here and there, and bodges where things end up doing stuff that’s different from what they started out doing (the three bones in your ear performing impedance matching started out as bones in the jaw).
Even so, I’m reluctant to add to the mess if there’s something I can co-opt that’s a better fit.
Historic=time_capsule it is, them. 
Thanks for the advice.
I have a problem with the time capsule I’m rendering. The Inscription field is too short. The plaque has the following inscription:
Oh Shenandoah, I long to see you!
“Big Apple Time Capsule”
Dedicated: Oct 19, 1990 - re-open in year 2040
Sponsor: Martinsburg Jaycees
This "Community Pride Project’ is an attempt to
preserve The Apple Capital City and surrounding areas
of our Southern and Shenandoah Valley heritage.
May God bless our endeavors.
The Inscription field doesn’t allow me to add “nd Shenandoah Valley heritage.
May God bless our endeavors.”
The description field has the same problem. Right now, I’ve put in as much as I can then added a note to refer to website for full inscription and description.
@Astromath yes, individual tags in OSM have a maximum value length of 255 characters.