Rome is among the easiest European cities to visit alone. A few specific things work differently, and one of them costs money.
What is easier alone
Museums and sites. Almost everything is single-ticket rather than group-priced, and a solo visitor moves faster through the Vatican and the Forum than any group does.
Last-minute slots. One person can frequently get into a timed slot that a group of four cannot.
Changing plans. Rome rewards wandering, and nobody has to agree.
Eating. Italian restaurants seat solo diners without comment, and lunch in particular is full of people eating alone. The coperto cover charge is per person, so there is no single penalty.
Standing at the bar for coffee — €1.20 rather than €4–6 seated — is normal and sociable.
What costs more
Accommodation. Single rooms are scarce in Italy and doubles are priced per room, so a solo traveller pays roughly what two would.
The mitigations: B&Bs and guesthouses, which are common in Rome and often have genuine single rooms; convent and religious guesthouses, which are cheap, central and quiet with occasional curfews; and hostels with private rooms.
Taxis, which split badly at one person. The metro, trams and buses handle almost everything.
Safety
Broadly fine, and the realistic risk is pickpocketing rather than anything violent.
The 64 bus between Termini and the Vatican is notorious. So is the area around Termini, and the crowds at the Colosseum and the Trevi.
Bag in front, phone away on the street, nothing in a back pocket.
Central Rome is busy and well lit late into the evening, and Trastevere and Monti are sociable rather than deserted.
Where meeting people actually happens
Food tours and cooking classes. Small, conversational by design, and the most reliable place to end up in a group.
Free walking tours, where people frequently continue to lunch afterwards.
Hostels with common areas, even if you book a private room — several Rome hostels are set up for this.
Aperitivo hour, roughly 6–8pm, when bars serve food with drinks and people are most open to conversation.
Trastevere and Monti in the evening.
The solo-friendly itinerary
What works well alone in Rome:
The free churches — San Luigi dei Francesi, Santa Maria del Popolo, San Clemente — which reward a slow, quiet visit more than a group one.
Long walks: the Aventine, Testaccio, the Appian Way by bicycle.
Ostia Antica, forty minutes away, where you can walk a Roman town almost alone.
The Borghese Gallery, whose compulsory two-hour slot means a calm gallery.
Early mornings — the Trevi before 7am, the Forum at opening — which are much easier to commit to alone.
Practical
Book the timed sites: Colosseum, Vatican, Borghese. Everything else is walk-in.
Water from the 2,500 nasoni street fountains.
Twelve to fifteen kilometres a day on cobbles. Good shoes matter more when nobody is suggesting a break.