In reply to BruceM: Hi, I was in Nepal last year for 6 months, and yes extending is possible, but it would and could take a full day out of your time waiting around.
First you need to find the tourism office, NOT the airport. The airport will do Visa upon arrival NOT afterwards....
The tourism office is on a side street off Ratna Park, once there, fill out forms, THEN pay extra to get it done same day....
If you run out whilst up at Lukla, you may find it hard to get on your flight, as everythiong is checked.
After last years experience, I'd say due to increasing tourism numbers, the pressure on the processes in Public Office are telling, and therefore clumsy and slow to say the least...
Do yourself a favour, pay the $90 and get a 90 day visa then you have no bother, piece of mind and one less thing to worry about....
On the positive, Nepal is still the beautiful, gracious and humbling place it's always been, and Im certainly jealous of your trip!
Moll