Beach suggestions very different from California

I have spent more time on the Italian coast than the California coast, so some more specifics would be helpful.
Are you looking for warm water then? Sandy only, or are rocky beaches fine as long as you can get some swimming in? It sounds to me like you are looking for charming coast more than necessarily a beach town.
Tuscany has some truly gorgeous coast in the south (the north has its charms, but is more resort-like), with a bonus of all the interesting hill towns close by. It would make sense to stop between the more typical Tuscany trip and Rome.
So, short answer is google "Maremma" and see if that floats your boat.

Would love warm water. Sand vs Pebble isn't important. True about charming coast vs beach town but we do want somewhere we can stay.

Why not Ischia, in the Bay of Naples? It's a short trip from Rome to Naples, a taxi ride to the docks, and a short hydrofoil trip to the beaches and spas (hot springs abound) of this ancient vacation spot. You won't have many Americans joining you, as the trade is likely to be mostly Germans and Northern Europeans, but it is an indolence and foodie mecca.

About Ischia, not sure that's our cup of tea -- from Wikipedia: The sharp increase of the population between 1950 and 1980 and the growing inflow of tourists (in 2010 over 4 million tourists visited the island for at least one day) have increased the anthropic pressure on the island. Significant acreage of land previously used for agriculture has been developed for the construction of houses and residential structures. Most of this development has taken place without any planning and building permission.[citation needed] As at the end of 2011, the island lacked the most basic system for sewage treatment; sewage is sent directly to the sea.[citation needed] In 2004 one of the five communities of the island commenced civil works to build a sewage treatment plant but since then the construction has not been completed and it is currently stopped.

We are so complicated! We like to go places that are interesting, but have a hard time with places that still exist solely because of tourism. That ends up taking a lot of places off our plate.

What time of year? (Or did I miss that?)

Good catch, Michael. Time of year matters.

In summer, Italian beaches are wall-to-wall people with reserved beach chairs and umbrellas. Nothing like CA. Outside of July and August, beaches are rather lightly attended. Liguria in October can be beautiful but the beach establishments are shut down and only a few Germans are lounging on the beach.

I second the Maremma as a possibility and encourage you to look at Porto Santo Stefano, between Rome and Tuscany on the coast. You can ferry out to Isola di Giglio (where the cruise ship ran aground) for some fine beaches. But not in July and August!

Going Mid to late June, returning July 3.

Late May and early June are perfect.
I stayed in a few places near the Maremma coast in late May last year. We chose Orbetello because we wanted to be in a town with lots of restaurants but be closer to the mainland for exploration (primarily to the Maremma park with its hiking and wild beaches). It is very unique--a lagoon town on a spit connecting a promontory to the mainland. Two adjacent spits feature sandy beaches. The promontory beaches are rocky and dramatic, and there are two villages (access for the ferry mentioned above).
Not far away on the mainland there are towns with interesting Etruscan heritage--lots of archaeological sites.
Of course being on the sea, these towns are not strangers to tourism, but it is mostly Italians and May is still early in the season. June will be better because of the greater chance for warm seas.

Italy (including the islands) has 5000 miles of coastline (longer than Seattle to Baja California), most of it gorgeous, so it would be difficult to suggest where. The region with the most beautiful beaches is probably the island of Sardinia, which is approximately 2 and half times the size of the big island in Hawaii, however all Italian islands, big and small, have gorgeous sea, as well as most areas of the coastline in the mainland.

However, since your itinerary seems to focus on Tuscany and Latium (Latium is the region where Rome is), I will limit my suggestions to those areas.

One of the best stretches of the Italian Coast are in southern Tuscany, an area that is called Maremma. That is your best bet for gorgeous waters in mainland Tuscany. Below is the official website for this area of Tuscany. Just click on the Maremma sea section. Porto Santo Stefano and Porto Garibaldi, in the Mt. Argentario peninsula (a national park also featured in the website below) are probably the best towns if you also intend to do some boating or ferry to the islands.
https://www.tuttomaremma.com/en/
Off the southern Tuscan Coast there are also several islands that are part of the Tuscan Archipelago. It’s a national park. You can see info below, although I can’t find the English page. But at least you can see the photos:
http://www.islepark.it

More to the south in Latium, near Rome, in my opinion, the only area that can rival the coast of Maremma is the rocky Ponziane or Pontine islands:
https://www.theguardian.com/travel/2016/may/23/italy-pontine-islands-ponza-ventotene

The gulf of Naples, in the region of Campania, also offers spectacular stretches of Coast, however that is probably outside of your itinerary scope, so I won’t mention it here.