Lots of people ask me what personal documents (that identify who I am) I travel with on a normal basis. Of course you have your usual passport, ID, etc., but it depends on the itinerary and where you are going to.
United States: for travelling within the U.S., I usually just take my Global Entry card which is an acceptable form of ID for TSA and airlines when travelling inside of the United States. Before I had global entry, I would use my driver’s license for domestic travel, but since the Global Entry card is a trusted-traveller document, I use it the most.
Within Poland/Schengen Area: while travelling within Poland or the Schengen Area, I use my Polish ID Card (different from Driver’s License), we have a special personal identity card which we use as an alternate to our passports and in addition, we also have a separate driver’s license. If I desire to drive in Slovakia (for example) then I would need both my Polish driver’s license and Polish ID card since Slovakia is a member of the Schengen Area.
Passport: I have both Polish and United States citizenships and therefore I can decided how to enter certain countries. Generally in the Europe/Africa/Asia region I will use my Polish passport since the visa requirements are lower and sometimes the cost of a visa is less with a European passport. For travel to Central America/South America I would use my American passport since I would be coming from the United States and not Europe (not that it really has to do with anything). For certain South American countries, I would use my Polish passport to avoid visas that Americans need. Plus, the Polish passport is just downright beautiful.
By car to Canada: Global Entry card works. Since I live in Seattle, I visit Canada every couple of times and therefore it is much more convenient to just take my Global Entry card rather than a passport.
Also, due to Polish regulations, I cannot identify myself as an American since I hold Polish citizenship and therefore I must enter the country as a Pole. Therefore when I enter the EU (usually in Frankfurt, Lufthansa First Class after all!), I always show my Polish passport and no questions are asked. When leaving the European Union, I always show firstly my Polish passport and then (if I’m headed to America), my U.S. passport. In fact a German officer in Frankfurt once told me that I should be proud to show my ‘European pride’ by showing first the Polish passport and then the American one – not the other way around. Which is what I have been doing anyway.
Hopefully that answers some of your guys’ questions regarding which personal ID/travel documents I use – it just depends on several factors!