travel writer37, IMHO Galleria Borghese is really the jewel (!!!!) of Villa Borghese and it's at the far southeastern end of the park (reference a map).
https://www.villaborghesetours.com/info/villa-borghese-gardens/. (for map only: ignore the tout for tours)
https://www.rometoolkit.com/walks/villa_borghese_walk.html
We've enjoyed just walking the (very large!) park: a nice bit of breathing room from the crowds around the top attractions. Especially nice is the view from the Terrazza del Pincio, which is at the far western end above Piazza del Popolo. Bikes and pedal surreys are available for rent at different points in the park if wanting to explore on wheels.
Editing to add, Santa Maria del Popolo, on the piazza of the same name, is one of Rome's must-do churches if you're an art lover.
http://www.santamariadelpopolo.it/it/?lang=it
Weekday: 7,30-12,30 / 16-19,00
Saturday: 7,30-19,00
Sunday: 7,30-13,30 / 16,30-19,00
No visits during mass
A little about the church (disregard hours):
http://www.sacred-destinations.com/italy/rome-santa-maria-del-popolo