Cell Phone?

The best is to use Google Fi. This enables you to use just one SIM and travel across Europe without changing SIMs and without outrageous roaming charge. You pay by usage and can stop service once you come back.

What are you planning to use the phone for? For example I use mine as a camera, a place where all my info is downloaded (maps, itineraries, confirmations), and occasionally as a device for calls and texting. So for me I need my specific phone - a burner won’t have a good camera or my personal data. I hardly make any calls to European numbers, most things I need to do locally can be done online or in person. That’s like restaurant reservations and such, I don’t have local friends I need to coordinate plans with.

Your needs will vary. But I think burner phones are a bad idea unless you need just the bare basic ability to make calls and texts. Getting your phone unlocked gives you the full use of your phone, minus your US number.

We have AT&T and for $10/day you do nothing and your phone, emails, everything works the same. It is the "International Day plan." We just used wifi some days and turned off the cell service.

If you are only planning to use your phone on wifi in the hotel, presumably you can do that with your current phone and SIM? What is your planned use for the SIM card? If it's just for emergencies, I'd just rely on my home carrier's per day plan. For me, that's Verizon, also $10 / day but only if you actually use it.

If you plan to use it while you're out sightseeing and not on wifi, then you might want local service, depending on the length of your trip.

I don't like having to waste vacation time on such details, so I bought a SIM at home that I take with me and use in my older phone.

If I'd be using my own phone anyway to take pictures, I'd put a local SIM in my existing phone I wouldn't have to juggle 2 phones.

ATT also has an international plan "passport" inclueds free text, some wifi, and a reducted rate for calls, about 50 bucks for 30 days. Some of the things you will need a phone for: QR codes for the restaurant's menus (even mom and pop places now use them). Covid documents and apps which your required to have eletronicaly and need to share your phone number the government incase someone on your flight later tests positive for Covid , Maps apps if you get turned around while your on foot even if you don't use the phone in the car. Translation apps. very helpful. I took my GPS, however the phone worked much better than the rental car GPS, or my Garmin. Enjoy your trip

If you buy a sim in Zagreb, you won’t need to put the phone in airplane mode and only use WiFi - you will have data available. For me in Croatia, that came out lots cheaper than my AT&T plan. But I suggest you have someone install it for you and get you set up.

I would either unlock your phone in the US or buy the cheapest unlocked phone you can fine. SIM cards in Zagreb only cost about $10 for a week of unlimited data and calls