We would advise you to:
Take with you possible gifts for schools ( e.g. pencils, exercise books, picture books ) - better than money
Take with you any spare walking gear you could leave with your guide / porter
Ensure you have adequate protection for your eyes and skin ( particularly if trekking high )
Use a wide-necked water container rather than a camelbak
Take a ' buff ' with you and / or a scarf
Take ear plugs for the barking dogs at night!
We would advise you not to:
Give to beggars - sadly, it encourages a dependency culture ... but it's hard to do
Buy plastic bottles of water
... and finally if you really want to send letters / cards home ask for advice on where to post them. Sadly the only reliable place to do so in Kathmandu, Pilgrim's Bookshop in Thamel, has burnt down!