Outr next destination was the magnificent Basilica of Saint Mary Major. Our next destination was the Basilica of Saint Mary Major.

the use of the adjective "magnificent" is technically correct, but if you are about to describe the location with verbs in the next few scentences, use the adjectives there instead.

For this one, we had to stand in line a bitfor (a bit/quite a while), but it was (so much/totally/absolutely) worth it! For this one, we had to stand in line for (a bit/quite a while), but it was (so/totally/absolutely) worth it!

For this one, we had to stand in line a bit, but it was so much worth it! For this one, we had to stand in line a bit, but it was so worth it!

"so much" is used for quantities, like "so much food" implies there's a lot of food. You don't use it for more abstract things like the feeling of something being worth it.

For this one, we had to stand in line a bit, but it was so much worth it! For this one, we had to stand in line a bit, but it was so worth it!

You can also use "absolutely" as another word of emphasis instead of "so".

The frescos wentre all over the walls, mesmerizing you with the beauty and meticulousness of the done workwork done. The frescos were all over the walls, mesmerizing you with the beauty and meticulousness of the work done.

or could say ...the frescos covered the walls entirely... for a bit more emphasis ...of the completed work. It just reads more naturally.

The frescos (went all over / covered) the walls, mesmerizing you(us/the viewer) with the beauty and meticulousness of the done work. The frescos (went all over / covered) the walls, mesmerizing (us/the viewer) with the beauty and meticulousness of the done work.

"went all over" is ok, but covered is more natural. It's a bit unnatural to use second person past tense here, as you don't know if the reader has been to Rome.

The frescoes went all over the walls, mesmerizing you with the beauty and meticulousness of the done work. The frescoes went all over the walls, mesmerizing you with the beauty and meticulousness of the work.

while technically correct, you can alternatively use this: "The frescoes decorated the walls, mesmerizing you with it's beauty." The "it's" refer to the frescoes, and while you can use "with it's beauty and meticulousness", keeping it simple can help avoid unecessary length.

The whole ambience of the place and its splendor justand splendor of the Basilica blew me away. The ambience and splendor of the Basilica blew me away.

ADefnitely/ It's a must-see if you're in Rome! Defnitely/ It's a must-see if you're in Rome!

At that point, we got both were hungry and tired, so we headed forto a restauraunt and ate some lasagna. At that point, we both were hungry and tired, so we headed to a restaurant and ate some lasagna.

I was surprised to learn that it's not really (that prevalent/customary) to ask for tap water in Italy. I was surprised to learn that it's not really (that prevalent/customary) to ask for tap water in Italy.

customary fits here a little better, nothing wrong with either

I was surprised to learn that it's not really prevalentcommon to ask for tap water in Italy. I was surprised to learn that it's not really common to ask for tap water in Italy.

"prevalent" is more used for natural phenomena like weather, or the spread of animals/plants/etc.

I was surprised to learn that it's not really prevalent to ask for tap water in Italy. I was surprised to learn that it's not really prevalent to ask for tap water in Italy.

The word "prevalent" leans more formal and while correct, you can use the word "common" or "normal".

In France, waiters bring tap water to you before you even get to ask them. In France, waiters bring tap water to you before you even ask them.

Although I've never been a huge fan of sparkling water, Italian "S.Pellegrino" made me change my mind! Although I've never been a huge fan of sparkling water, Italian "S.Pellegrino" made me change my mind!

San Pell does slap