The 2007-9 Financial Crisis turned Iceland into a major tourist destination, as a newly cheap currency combined with affordable flights and natural beauty. For anyone with plenty of time and a moderate amount of money, chasing the newly-cheap destination seems like a good travel strategy.
Since January 2020, here are the countries where the US dollar has gained the most vs the local currency:
Of course, in many of these countries the local currency has weakened in real terms (higher prices in the country) as well as against the dollar. After taking this local inflation into account it is probably Turkey or Brazil, rather than Argentina, that is cheapest compared to where it was before global travel shut down. This is convenient because Turkey and Brazil are currently accepting Americans while Argentina is not (though note that the US State department currently recommends against visiting Turkey or Brazil). I’m most excited to visit Turkey and see ancient sites like Gobekli Tepe (though as someone with two young kids and a higher-than-average chance of winding up in a Turkish prison, I’ll stick to visiting family in the US for now).
The above calculations show which countries have gotten cheaper since before the pandemic- but if you weren’t doing tons of global travel before, you probably care more about which countries are cheapest overall, regardless of how cheap they were before. The Economist’s Big Mac Index is a good way to get at this- it shows that the Lebanese Pound is the most undervalued relative to the dollar (68%), with Russia and Turkey close behind. But if you are feeling more cautious and want to go to a country that is allowing Americans, has no State Department travel warnings, you have to go way up the list to Sri Lanka, whose Rupee the Economist reports to be a mere 34% undervalued. Still, if you have the chance to safely and legally travel abroad this year, the US Dollar is almost certainly your friend- the Economist reports that only 3 currencies (Switzerland, Sweden, and Norway) are overvalued relative to the dollar.