I dug out this report I’d written about a visit to Rome’s Ciampino (low cost) airport last year. You may find it useful for spotting:

Ciampino is closer to the centre of Rome. It acts as the main low-cost and
business jet airport, and is also a military base and home of the
government’s fleet of A319s and business jets. It is also home of the yellow
CL-215 aircraft.

The terminal building is quite compact, split into Arrivals and Departures
sections. Passengers only use the ground floor throughout, and buses take
you to and from the aircraft.

Once through to departures, there is a separate section for Non-Schengen
flights. Hold off from going through there until you are due to leave if you
are a spotter, as the departures lounge for Schengen flights has better
windows looking out to the apron, taxiway and runway. Most aircraft park
near the arrivals section, however.

Outside the terminal, beware of the large presence of police and military
personnel. Cameras and poles will not be looked kindly upon. You can,
however, log most aircraft on the cargo/biz jet apron if you are discrete
with your binoculars. Walk to the car park next to the General Aviation
Terminal, and you can see all of the aircraft through the fence. Police do
patrols, so don’t loiter.

If you have a car, you can drive to both ends of the runway – again, don’t
loiter. At the southern end (turn left out of the airport entrance, and then
left at the roundabout), you will see the cargo aircraft close up, and a
small compound of derelict biz jets.

The hotel Palacavicci is situated at the northern end of the runway (right
out of the airport entrance, then right again towards Ciampino). I stayed in
room 606 and had uninterrupted views of aircraft just before touchdown.
Rooms in the 7xx and 8xx range should also have similar views. If not, you
can wander the car park to spot anyway.