1. Firstly, get to Nepal through Delhi because there aren't any direct flights from Colombo at all. CMB to DLH is about Rs. 50,000 +/- and DLH to Kathmandu would be about Rs. 15,000 on a budget flight like Indigo. (Indigo is the most AWESOME budget airline EVER). Get a data sim at the airport.
When you get to Nepal, you'll probably hang out somewhere in Kathmandu. It's actually a lot like Sri Lanka!
3. Next, go to a Himalayan viewpoint. It's best to view the sunrise of sunset. Nargarkot takes about 1 hour from Kathmandu and is really beautiful (and bloody cold). There are about 5 such viewpoints to visit if you like.
4. People watch. Nepalese people are sweet and a lot like Sri Lankans. The cities are very commercial though, even the temples. I didn't really get the peaceful feeling when I visited a lot of them...they even have restaurants and trinket shops inside the temples sometimes. Apart from that, there's a LOT of beautiful wood and metal work and art to go ape over. (If that's your thing)
5. Get to Pokhara. It's like the Kandy/ Nuwara Eliya of Nepal. Its about 300KM but takes about 5-6 hours in a rented car and Rs. 15,000 for about 4-5 passengers. I'm told you can fly there for about Rs. 5000pp...not sure about that though.
In Pohkara? Great. Try Paragliding. You literally run and jump off the face of a mountain. The parachute drags you back but you have to keep going, hoping for the best. The initial flight is terrifying but you get your air legs back and it's quite rad. A must try. One trip would cost about $90 +/-
6. Visit Davis Falls in Pohkara, visit the adjoining cave of Davis Falls. It's quite magical. Davis? A Swiss lady who died there. :(
SOS Children's Village.
7. Back in Kathmandu? Visit Bhakthapur/ Durbar Square, a 17th century royal square where ancestors of those who built it still live today. Get a guide for about Rs. 500-1000 and get him to take you around. The Karma Sutra carvings are ahem!... (All these sights are quite close by, so take a cab from the street and haggle for a acceptable price! DO NOT take hotel cars... Rs. 3000 IS a rip off)
8. Buy a Thanka and a Singing Bowl. Thankas are traditional Tibetian/ Nepali/ Buddhist art. No, I didn't buy the one below.
9. Visit Swayambhunath and Boudhanath temples - ancient sites sacred to Newar Buddhists and Hindus alike. Remember to read up on the history of the temples, you don't get guides there. They are also close together and a street cab is the best bet.
Well, those are my tips for a short trip to Nepal. Of course there's a million other things you CAN do, like trekking, volunteering, travelling to other cities, bungee jumping, going native or smoking up (yes, it's everywhere and very pungent). I really think we only scratched the surface of Nepal and did a lot of touristy things and there's a lot more to discover.
Oh and the food? Looks a lot like Sri Lankan/ Indian cuisine but about 5 shades blander. Not something to crave after :)