First Sunday in Rome -- How to navigate in/around free but overcrowded sights?

The Galleria Borghese is always limited to 300 people per visit, so it never gets over crowded. After that consider walking tours that do not go into museums. DIY using Rick Steves’ audio tours or the ones in his book, or with a guide, like Walks of Italy.

In the evening, take in a night tour such as http://www.viaggioneifori.it/en/.

We were just in Rome on the 1st Sunday of May. We chose to do a less prominent but really interesting museum , Museo Nazionale Etrusco di Villa Giulia, and it worked out pretty well. We went just after it opened and it was not crowded. By the time we finished, just before 1pm, it was very busy, but not to an uncomfortable level. Most of the other patrons seemed to be Italian, especially families, so every one was pretty polite. There were almost no group tours so there was none of the rude herd effect.

We did one day visiting Churches. We started with St. Paul Outside the Wall, St. John Lateran, St. Peter in Chains, and Basilica Santa Maria Maggiore. There are many, many other Churches just as beautiful, they rival any Museum. Another place we stopped at that wasn’t crowded was the Doria Pamphilj Gallery on Via Del Corso, near the Vittorio Emanuele monument. It had amazing art work and the family still lives in the home. It looks very unassuming from the outside, but is magnificent inside.

Does anyone have experience with Ostia Antica on the first Sunday of the month? (as far as how the crowds are?)

A surprising number of sights in Rome aren't state-owned and don't participate in the free first Sunday. As suggested by others, you could visit those sights, or there are a few state-owned sights I've found not to be overcrowded. I've visited the National Museum twice on a free Sunday and it wasn't overcrowded either time. I've also visited the Forum and Palantine Hill on a free Sunday afternoon. The wait to get in was short and I didn't find them to be overcrowded. The Colosseum however had very long lines.

I visited Ostia Antica on the first Sunday in June 2016. We arrived around 10 AM and it was not crowded at all, although i do remember that the cafeteria was packed around lunch time. I would definitely recommend it as a first Sunday site to visit.

Ostia Antica is too large for it to get crowded, even at the busiest days. There is too much space for people to roam and the sight itself is nowhere as famous as Pompei (which does get very crowded on summer weekends).

Thank you!

I was in Rome last month, and we had a small group evening tour of the Colosseum pre-booked. it was fantastic, and we didn't notice anything more crowded than usual.