Hi! I'm "assuming" that you will be docking at the cruise terminal that's by Pan Pacific hotel.
If this is the case, you will be very close (across the street from) the
"Tourism Vancouver Visitor Centre" (Plaza Level, 200 Burrard Street, Tel: 604-682-2222). They can help you with ideas for your 1 day visit, give you maps, coupons, brochures and direct you to the nearest postal outlet.
In the meantime, you can start planning your visit (ie. get ideas) by visiting their website at the following link:
https://www.tourismvancouver.com/
* Tip: Use google maps to check out distances (ie. walking, driving).
If you can't make it to the Tourism Vancouver office for information, there is a postal outlet at the following location:
Postal Outlet: Rexall Drugstore (499 Granville Street / Tel: (604) 801-6991))
* This Rexall is about a 8 - 10 minute walk from Pan Pacific Hotel (again assuming that you are docking at this cruise ship terminal).
* Tip: ask them to "weigh" your mail item & they can advise "exactly" how much postage you will need. If you don't have enough stamps you can purchase the remainder.
The above noted postal outlet is just one of several that you can walk to from the Pan Pacific Cruise Ship Terminal ... perhaps Tourism Vancouver can direct you to a "closer" location if there is one that's closer.
Good luck and enjoy your visit!