Extending a Visa:
Getting the visa exteded in Kathmandu is fairly painless. The immigration office is on Durbar Marg, across the marching field from the Kathmandu Mall (N27 42.119 E85 19.027). You fill in your visa application form, which a couple of guys there will help you fill out. Stand in line at one counter where they take the form and passport, and give you a receipt. With this receipt you pay your 2000Nrs at another counter, and you can get your visa in 3 hours or so. The time is noted on the receipt that you get.
The guys who helped you fill in the application form, will, for a fee promise to return your passport in 10 to 15 minutes!
The visa will be extended for a maximum of 30 days at a time, and they literally count the days.
Extending a Carnet:
In order to extend the carnet, you will need to go to the airport cargo/customs building (N27 41.417 E85 21.173). If you approach the airport from the south it will be just north of the juction, around 1km south of the Airport entrance. If you come from the north, just pass the airport and the building will be on your left 1km further down. They don't start work until 10.30 or so, and the head customs guy doesn't show up until around 11.30/12.00, so don't bother going early.
You will need to see the chief customs officer, he has an office upstairs, and one downstairs, so pick one and explain what you require. They will track him down and he will then send the document around to get some signatures, and a stamp and you are on your way. They will need to see your passport, and will extend the canet to the date of your visa. The time I did this, it was news to them that there was a 60 day limit on it, and insisted that customs officer at the border had done it wrong. The actual lenght being 6 months. Of course they weren't willing to extend it that long themselves. And extending the visa for more than 30 days is also not possible.
The whole procedure from the time the chief customs officer got there to the time I got the document back in my hands was 1 1/2 hour. So bringing something to read isn't a bad idea, as you will just wait while someone runs around getting the relevant information.
You can change your Indian rupees at a set rate (1.6 to 1, see below), at the "Change" booths just down from the Nepali immigration office. The Nepali rupee (Nrs) is pegged to the Indian rupee (Inr) so the change offices will charge a comission or surcharge. The rate is 1.6 Nepali to 1 Indian rupee. Of course all other manner of currency transactions are also available at these booths.
On leaving make sure that you get rid of your Nepali rupees.
At the airport: Other than the airport tax, there are no other fees or costs involved. The waiting lounge area has a snack bar and duty free as well as internet and kiosk, and a bank (which is NOT open for the early flights), so do it downstairs where you paid the airport tax.