I typically take my Tamron 17-50/2.8 + DA 55-300 + one other not-too-big lens that I might use if I feel all 'artistic'. The artistic lens taken can vary depending on what mood I'm in and what I think I might encounter. Last trip it was a FA 43 Limited. In practice, 90% plus of images use the Tamron, 55-300 comes out if critters are involved or I'm plane spotting at airports (although I got some great night shots of KL last trip with the 55-300 on a tripod) and maybe just once the 'artistic lens' gets used. Sawpping lenses a lot can be impractical (not enough spare hands, being hurried along, dust worries), so I find I don't do a whole lot of lens changes.
This lot works well for me either on round the world business travel or regional family holidays.
I'm off again in two weeks for work. Umming about taking a FA20-35/4 in lieu of the Tamron as I'm grown to rather like the 20-35 (sharpness + colors) of late. But Tamron does better star bursts for city night scenes and I do like my night shots. But for dull interiors, the F2.8 vs F4 has its advantages when ISO is already being pushed up. The 17-20 bit of the Tamron also has its advantages in cramped settings.
A 60-250 is bulky and heavy. Not so good for travel/walking about lots. That is the 55-300 advantage - you'll carry it about everywhere while a heavy lens will spend days sitting in the hotel room.
WR can be over-emphasised. Be half sensible and water ingress shouldn't be a problem (stand under an eave/tree, stuff camera under waterproof jacket if caught outdoors, etc. If using a zoom, wipe droplets of extended barrel before retracting.
Last three trips I've packed a tripod so I can get city night shots. I get by with one less pair of shoes now to make room / offset the weight. I use a Cullman 4 section tripod with Manfrotto ball head and RC2 quick release plate. If I travel with just a small carry-on suitcase, I unscrew the ball head and the whole lot fits neatly in the carry-on. For a long trip with larger suitcase, ball head on/off doesn't matter. Might only get used once or twice, but the photos are worth the effort.
Some teaser images from Stockholm last year (to bolster my case to take a tripod):
Stockholm at Night | Photo Morsels
Haven't choosen the artistic lens for the coming trip. A 31 limited seems to be at the top of mind at present.