I love to use old and discontinued Canon EF-compatible lenses. Sometimes I hunt for bargain lens at the flea market, but mostly I buy them at Ebay.
Old lenses still offer fantastic performance for a fraction of cost, but what is even more important the old lenses allow to travel light (literally, in weight terms)and without worrying about costly equipment being broken or stolen. In a perfect world without budget and weight constraints, I would travel with Canon EF 16-35mm f/4L IS USM Lens, weight 616g, retail price about $1,000. Yet I compromise for the old working horses like Canon EF 20-35mm f/3.5-4.5 USM or Tokina AF 19-35mm F3.5-4.5 for Canon EF, weight about 340-450g, prices at ebay $150-$180.
If I have to travel with only one prime lens, I choose between discontinued Canon EF 24mm f/2.8, Canon EF 28mm f/2.8 , Canon EF 35mm f/2 or new Canon EF 40mm f/2.8 STM Lens, with 28mm f/2.8 being the lightest (second only to new Canon EF 40mm f/2.8 STM) and most universal.