I took my (now ex) girlfriend up once, next time I take anyone up I'll vet them very carefully as it was a complete nightmare. I hadn't passed my test so she was in the back. At only 5' tall she didn't find it cramped, but remarked the interior was a lot smaller than she was expecting.
She gave no indication she'd ever had any kind of motion sickness before - having bungee jumped, I thought this was well within her stride. How wrong I was.
Twenty minutes into the flight we were circling our hometown with occasional light turns. While the instructor and I were pointing out landmarks she said she felt ill.. . So we said we'd forget the sightseeing and just fly home. Too late.
The worried looking flight instructor produced a very crumpled, worn airline style sickbag that almost looked like a screwed up ball of paper. She passed it back to my girlfriend who made immediate use of it.
I felt sorry for her, but I'll never forget those sounds coming through the headset. I'll also never forget the look the instructor shared with me, (not my usual instructor either) a look of mutual repulsion.
We got back as quick as we could, but unfortunately had to go-around due to a slow microlight on final. This only contributed to my girlfriend's wincing and moaning.
In all, it was a deeply unpleasant experience for all involved but my girlfriend managed extremely well - no mess and no smells! That crumpled up old sickbag did it's job.
What did I learn? How important it is to enquire with intended passengers what the likelihood of them getting motion sickness is... Have they had it before etc...
It was such a beautiful day that day, also, it was the last time I ever flew that aircraft as I moved house.
I wish you luck overcoming this issue. Some fine advice above from the others.