Nepal has a lot to offer when it comes to food. From high-end to the street, you will find a diverse range of cuisine. Though street food can easily come under $5, it is up to you to face the hygiene risks that come with it.
The typical Nepali dish is the Dal-Bhat, which is rice and lentils with some curry and pickles. This carb-packed dish is what the locals dine on and what powers the trekkers through their arduous journeys. Cows are not consumed because of their religious status in Nepal, but you can find imported meat in restaurants. You will easily find Italian or American if Dal-bhat becomes a bit too mundane for you.
Bottled mineral water is usually readily available. Do not drink tap water, and use chlorine tablets if you have to use tap water during your treks. Beer and other imported liquor can be easily found in bars and pubs in the capital. You will also be able to consume homemade rice ale chyaang if you are trekking in the Himalayan regions.
Some other dishes and drinks to try are:
Newari dishes: Choyla, sekuwa, chatamari, yomari, etc.
Cel-roti: rice flour doughnut made especially during Tihar, a major Hindu festival
Momo: dumplings filled with meat or vegetables
Ju-JuDhau: Specialty curd from Bhaktapur
Lassi: Sweetened drink made from curd