Get fit before you go, some people find the steps tough going, but most days are no harder than a big Lake District walk. Toilets are basic, but you get used to that, showers are interesting, so be prepared for a quick cold wash some days.
Most people had tummy upsets at some stage, so immodium was very welcome. Food was generally good considering that it's all carried up there. Dal Bhat is always reliable. Sleeping was better than most alpine dorms, with rarely more than 3 to a room, but take wax earplugs, walls are very thin!
Get up before sunrise and enjoy the mornings, after all the sun rises at 6 am ish, and you have to be ready to walk by 7.30, so don't waste the sunrises, especially if you like photography. You can charge camera batteries usually, but take a spare and keep them charged. It's cold at the top, so sleep with your batteries.
Take a small umbrella, if there rain it's easier to use than drying off wet gear in the evenings, it's also a sunshade if its hot.
Poon hill sunrise is great, but very busy as many treks converge here. Tadapani has a great sunrise too and there were only a handfull of people enjoying it.
Trekking is big business in Nepal and there are a lot of trekkers. The paths are the main roads in the mountains, so expect traffic, porters, trekkers, donkeys, it's all part of everyday life, so enjoy it!
Nepal is poor and the infrastructure creaks at times, so be patient.
Kathmandu is an experience, very crowded and at times difficult to cope with, but also vibrant, colourful and massively atmospheric. Walk to Durbar square, visit Bhaktapur outside the main city. An oasis of calm and a window into the past. The guide can organise the bus for this.
Photography wise, I took an SLR and a tripod for sunrises etc, 4 batteries and 96gb of memory. Use a good zoom, say 18-200, a polariser in the mountains. Shoot panoramics too. In town, use a fast prime lens.