I haven't often travelled. I've been on holiday twice with my parents, for week long trips to Spain that were spent mainly around the hotel (and I forgot the other name for the kind of place we stayed). We had a couple of day trips each time. I coped because my parents were there to basically control everything. Also, my dad's doesn't care for activities that much so he would often go back to the room to nap, and so the holiday wasn't too stressful for anyone (except my mum who gets bored and would rather be doing things).
Last year I travelled twice with R, to Glasgow. The first time we flew and she made me a lanyard that had 'ATTENTION' on it and informed people have I have autism, because I can't talk to strangers and can act strange when I'm anxious or stressed. We foresaw that I would struggle with the airport, and my main worry was being separated from her and not being able to seek help, or rousing the suspicions of security and possibly being frisked. I'm not sure if the lanyard was what did it, but I wasn't frisked at any point and the security staff were very nice to me.
We stayed in a Premier Inn and initially I was very homesick on first arriving in Scotland and things were distressing me quite easily. I left London in sunshine, arrived in dismal Scotland, so I was immediately quite anxious, and then the cash machine gave me Scottish notes and we had to get a bus outside the airport to the Premier Inn. We got to the Premier Inn and I didn't like that I was stuck there for a couple of evenings, and I got cold and the heater wasn't working. R got that sorted very quickly, thankfully. We took books and DVDs and things from home so that I would feel more comfortable and less panicked about being away from home. Keeping busy and then being able to come back and chill out was nice, but I think that it helped that R is someone I'm very comfortable with and I don't have to feel too embarrassed about melting down in front of her.
We had a map of the immediate area and R had researched and written down details of things that might be important if we wanted to visit anywhere that required using the underground or getting a bus.
We flew back and that was even better because I was quite excited about going home, even though I had enjoyed myself. Again, I had my lanyard and people left me alone or were nice to me. I felt less pressured knowing that if someone tried to talk to me I could give them my lanyard to read, and that if I got separated from someone I could go hand them my lanyard and they'd contact R for me. So that helped with my anxiety. I also had books and things to distract me so that I didn't have to be overly aware of the airport, although it wasn't as crowded as it might have been for a bigger flight (Glasgow is in the UK, after all, so it's a domestic flight).
The second time we got the train and R made sure to get us a seat near the toilet because I can't walk through carriages to find a toilet by myself, but she wouldn't have been able to come with me because we couldn't leave our stuff unattended. So when we arrived she explained that I'm disabled, and got us seats near the toilet, and although it was a long train journey I quite enjoy things like that and had lots of things with me to keep me occupied. We also had seats alone so that no strangers sat with us.
We stayed in the same place, which was helpful. Different room, which threw me at first for some reason, but arguably a better one I suppose so I settled in a little quicker this time, and the heater worked fine. We stayed for longer and although we had trips to keep us busy and a map to help us find places. The area was familiar but we had our maps again, to help us. We visited Edinburgh Zoo on one of the days, but that was a bit overwhelming because a friend came and whereas I'm very comfortable with R I was less comfortable with the friend and was expected to talk more than I normally would! On the way back we sat at the back of the bus and I was hot and tired and then I had a meltdown over a change in plans (the plan had been to get back to Glasgow, get food, R and the friend decided they weren't hungry and even though I wasn't I couldn't cope with the change in plan and panicked about what would happen about getting food, and in the end I went back to the room by myself for a bit to chill out and calm down and R and the friend went to get food (and got some for me). That was harder because my friend became aggressive in turn (i.e. she tried to hug me or poke me or something as a sort of, "What's up? Cheer up!" sort of thing, and I snapped at her not to touch me because I was obviously hypersensitive and less able to tolerate being touched than I'd usually be and she snapped back, "Don't get pissy with me!" or something along those lines which annoyed me even more because I was feeling quite embarrassed and horrible without also being told off for it!).
Annnnd then it turned out that R had booked the wrong tickets or got them mixed up and we got stranded in Edinburgh on the way home, and had to stay overnight in a Holiday Inn. I had a meltdown at the train station, then in the cab, at the airport, then again at the Holiday Inn. I hated it because it was different (Premier Inn = purple and white rooms, Holiday Inn had a sort of burnt orange and white theme), I was in Edinburgh instead of Glasgow, I had expected to be on my way home or at home instead of in another inn sort of place, and I was tired and miserable and had been really stressed and anxious the whole time over what was happening and had no control over being able to get home, so... Yeah... I actually got quite sick from the stress, developed a fever and a sore throat, and R went out to get me some things to help with that and to cheer me up, and then came back and we got ready for bed and she put a DVD on for me, and that did actually make me feel better (goodies, a familiar film to distract me, and the knowledge that in the morning I could get the train home).
You can't prepare for every eventuality, I suppose, but I would suggest maps and trying to familiarise yourself with the area straight away (exploring, but maybe gradually, so walking around the area you're staying to see what's nearby, what food options there are, where you can get necessities, and to help you get your bearings so that if you did want to venture out without your friend you'd be better able to find your way around (even if you can't venture too far by yourself). As it's another country, maybe learn some key phrases in Italian, even if it's just so you can ask where toilets are or something like that, and maybe work out where you'll be visiting and how to reach those places from your 'base'. If you can, take some things that are familiar and comforting (probably better if they're also entertaining for you to keep you distracted, a book, sketchbook, DVD, handheld games console, something you can look at that will help avert your gaze from crowded areas). Maybe locate any places that might be nice to chill in if you need some time-out to avoid a meltdown (either a room you can use by yourself, or headphones so you can share the space with your friend but be left alone, or a nearby park or something). If you are sharing a room with your friend and there's a bathroom, perhaps disappearing to shower could give you some time to yourself as well.
Make sure you get things in or have things on hand like foods you like and drinks, because being hungry/thirsty when in an unfamiliar place makes the stress build up a whole lot worse than being hungry/thirsty at home!