"I would like to bring along my tube of diclofenac gel (available over the counter) but the tube is so darn big!"
Depending on where you are going, you may be able to get this pretty inexpensively in a European pharmacy. Before it was OTC in the US I got a big tube of Voltaren-branded gel in a pharmacy in Germany. I don't know how much that tube is in the US but 7-8 years ago I paid about $15 for the giant size.
I don't have knee problems but have traveled with my brother who does (currently looking at him lying on the couch with a cold pack on his new knee, lol). Even if you are not having problems when you leave, do take the above mentioned neoprene sleeve in case you have issues. My brother started upping his knee exercises in Dec when scheduled for a total knee and added in some new ones he found on youtube. He was pleasantly surprised that he was able to increase strength in his quads and his knee felt much more stable before the surgery. This might help your knee stability.
I'll add that for my balance I do exercises on a BOSU. This is the half ball on a platform that is a balance trainer. There are some much smaller ones than this one which I've had for years. I can really tell a difference when I use it regularly before I travel. Strengthens my feet, ankles and knees (and probably core) for cobblestone walking.
I have had bouts of plantar fasciitis in the past but switched to zero drop minimalist shoes. I know these are not for everyone but the proprioception benefits of a softer sole are great. Even hiking I like the way I can feel the ground better than what I could with thicker soled shoes.
YMMV.....