I-phone charger

If you mean a plug in wall charger, it is very likely dual voltage. So all you will need is a plug adapter so you can plug it in the socket, like this one https://store.ricksteves.com/shop/p/uk-power-adapter

It depends. If you use a plug with your charger, you will need an adaptor. If you just plug it into a USB port, you do not.

That said, many hotels now have USB ports for travelers to use, but I wouldn't count on it, so chances are you will need to bring a plug (or a power strip, like this one). And you will need an adaptor with that.

I was in Ireland in May and used this adapter with my iphone charger at all my hotels:

https://www.amazon.com/dp/B01MZD5T3E?ref_=ppx_hzsearch_conn_dt_b_fed_asin_title_3&th=1

Something like this is one piece and universal:
https://www.amazon.com/NEWVANGA-International-Universal-Worldwide-Charging/dp/B01FO4W5W2/ref=sr_1_3?crid=3DJC83L9NH6BQ&dib=eyJ2IjoiMSJ9.-zkc_-bjoO8zJD3SI4XHIKqa7_dkwW45HZ5NP_ma28bZ0zXaChcczmpa4S-JJULaNqfDJ5PH2mmcnx3f4qS-9HRq1VowDuN2pgpQS4z1hM-6QmhNct7L1JGn1BFRDZDykYoJkPuIDlotoSURNY2fH0y5jI41UraCB_2am9gm_xIMe4YGDywrj2kREOnIY9TwATbE13rFF5MopovQ5N_v8uWbsa822HVPAzB3nJZAX2puRKu5rFX0eVIHfpffMnc81ucBmsq6Ka0HWUHF0JU1oYP4QNiDptPDyGBqdOZH2DE.qnQRodzPCSdSEXTO5xM7t8HdLXzHaqoBgr8nYwl6WlM&dib_tag=se&keywords=universal%2Btravel%2Badapter&qid=1753480848&s=hi&sprefix=universal%2Ctools%2C167&sr=1-3&th=1

I also use the type of adaptor that plugs into the wall (like the universal one in the Amazon link someone else provided). I’ve had more than one occasion when the USB port provided in the hotel room didn’t do the job but the plug in one did fine. I also recommend the Anker power bank that attaches magnetically to the iPhone (but be sure it’s compatible with your phone case). It’s so great if you use your iPhone for navigation, which drains batteries big time, without running the risk of not having power left for those tickets you need to access.

What other things will you need to charge?

my I-watch and my tablet

The same adapter for the iphone will also work for your apple watch and ipad.