Croatia is currently the only member of the European Union that’s letting in US citizens as tourists. However, the country has begun requiring those arriving by air and land to provide a negative covid-19 test that’s less than two days old and proof of booked accommodations. Other countries in the region that show similar benevolence to Americans are Serbia, Kosovo, and Albania.
Americans have been banned from entering other EU countries or are given rare exceptions to stay in them, but with the obligatory departure to 14-day self-isolation. The Croatian National Tourist Board reports that just over 900 American tourists are currently staying in Croatia.