Some people are happy to carry little potties with them when they go out, but I like to travel rather minimalistic - a baby, a sling, a bag and off we go!
I developed a very portable potty system (VPPS!) for use with my DD when we were out and about, including on planes (longest flight leg was 12 hours) and buses (longest trip was 16 hours).
In a zip-lock plastic bag I had a face-washer or wash cloth (small towelling square), a small container (500 ml margarine container) and a 375 ml screw top (plastic) jar. The jar fitted inside the container when not in use.
The sunglasses in the picture are just for an indication of scale!
When my little one needed to use the potty, I took the VPPS out of my bag, sat down where ever we were (gutter, park bench, seat in the shopping centre, seat at the bus stop etc), placed the container between my thighs, pulled her pants down and held her in position over the container. I cued her and she went.
It took a little practice to get the right angle of the container so that it didn't spill forward onto my legs :-) When she was finished I stood her up beside me, pulling up her panties and pants at the same time. I held tightly to the container with my legs at this stage. Then when she was stable I tipped the contents of the container into the screwtop jar, put its lid on, wrapped it in the face-washer in case of any leakage, popped it all back in the plastic bag and off we went. I emptied it into a toilet when I had the opportunity.
It seems complicated when I write it out like this, but using this system took us very little time. It was discreet enough to use while we were travelling in Turkey on public transport and even while waiting in Singapore's Changi Airport.
We used this system from the time that my daughter was about 7 months old and I got brave enough to take her out of nappies when we were out of the house. Its final use was during another long overseas trip when she was 23-25 months old. She reliably told me in advance of her need to use the toilet from the day we left on that trip, so after our return home I retired the VPPS and since then we have always used either public toilets or the road verge or similar places in an "emergency".