A bit too early to say.

The European Commission will come up soon with recommendations to ease travel around Europe, but as usual it is up to countries to decide. Italy and Greece are likely to open up for tourists in May, with Spain to follow soon.

So I think you could do something like US to Greece or Italy. Traveling between states will be more difficult (likely including at least CoVid tests) but potentially possible within Schengen areas w/o quarantine. Most likely you won't require tests if you can prove you are fully vaccinated.

I think the UK might be the most difficult bit as the US is on the Amber list, and most of the EU will remain on Amber until end of July (my bet) if not later. There are rumors that there might be a travel corridor between the US/UK, but nothing which is really confirmed.

I would plan a rather simple itinerary from the US to either Italy, Greece, or France without the combination of those. Travelling between these could be possible as they are already mentioned they'll be open to tourists, but you need to be prepared that your plans might change somehow.