I don't buy "travel clothes", but I keep travel needs in mind with most of the clothes I buy. That means a) a bit of lycra; b) colours that work with a base of black or brown (I alternate base colours for travel); c) lightweight fabrics that breathe, hold warmth and pack down tight; and d) don't need ironing.
En route and often at my destination, I frequently wear leggings, tunics and either ankle boots (fall, winter, spring) or comfortable walking sandals (summer). I get most of my tunics and leggings at Boden. They're dressy enough to go out to dinner (except super-formal restaurants) and comfortable enough to sleep in on planes or lounge around in at my hotel. They also take up very little space.
Except in winter, I usually travel with a lightweight crushable car coat length raincoat (from rainforest) and a mid-thigh length cardigan. I can wear one or the other, or combine them for warmth. The coat works with office clothes but also works well with casual clothes.
I also usually pack a couple of knit dresses and wear them with a short wrap merino cardigan. My favourite stores for these are Boden (great wrap dresses that stay wrapped!), Title Nine, Athleta and occasionally Ann Taylor.
If I'm travelling for work, I usually pack one pantsuit and then wear dresses for less formal days.
Finally, I usually pack a pair of Old Navy chinos (which have just enough lyrca to be comfortable and not so much that I look like a sausage) and a couple of white and/or black tshirts. They pair up with the wrap cardigans.
I try to stick to 3 pairs of shoes in total (including the ones I travel in) in the winter (maybe 4 pairs in summer because sandals tend to take up less space). Since I work out when I travel, that means one pair of runners (charcoal grey/lilac Nikes - not too dorky for travel days or sightseeing), one pair of 1.5" heels for work/dresses, and a pair of ankle boots, walking shoes or sandals.
I'm not sure if your trip has started yet, but one thing I wanted to mention is that I've lived in both Paris and London, and found that neither was as cold as I expected. I did find that it was critical to layer in the fall, winter and spring, because it can be brisk outside, but subways, restaurants and theatres can be stiflingly hot. The raincoat + long cardigan combo has worked for me except in the dead of winter in places like Paris and London, provided that I supplement it with a pashmina and a pair of gloves.