I've been to Belize, the Turks and Caicos, the Dominican Republic and Honduras... and Mexico on cruises and we took a couple little road trips through a little of south-central Canada when I was a teenager...but that's it.
I hope to at least knock out all the full 47-50ish NPs (they've added a few in the last few years so I'm not sure the exact number) in the lower 48 before I croak..but I've only been to 23 (edit: 24, I forgot Mesa Verde...Edit #2: 25, I forgot that they've just turned Gateway Arch into a full NP in 2018 and we've been there twice since 2011) full NPs so far, so I still have over half of them to go.
We have also been to a ton of the top State Parks in the states we have visited, and a ton of the National Monuments, NRRAs, National Battlefields, National Sea-lakeshore and National Historic parks which are in the NPS system...and a lot of the popular National Forest sites which are in the NFS system...which are highly underrated and underfinaced imo.
We are going to 4 new countries on a 10 day cruise we are taking in November...Costa Rica, Panama, Cayman Islands and Cartagena Columbia...so I will be able to say I've been to another continent.
Ps: IMO...Gateway Arch, while really cool, and while I think everyone should check it and the museums on-site out...should not be a full NP..
There are several AWESOME scenic areas among our federal lands that should have been made a full NP before that place.