One thing I believe is so important is manners, they cost nothing (well maybe a postage stamp) and this is something I am keen to teach my child about... maybe I am old fashioned?
Amidst the manic present opening session on Christmas morning I had my phone by my side (I had aimed to have a day away from my phone!) and wrote down a little note of all the gifts we had been given and who they were from so that we could write Thank You letters in the New Year. I also use this tactic on birthdays as it is so easy to forgot what everyones gifted and I hate just sending a generic 'thank you for my lovely present' message.
I then use sending Thank You letters as an activity for my little one, from creating / decorating the 'letters' to putting a stamp and address on the envelope to hand delivering or posting them. I find my little one loves to get involved in each stage of the process. I also believe emails and text messages have a place for Thank You's especially if time is more limited. I just feel the most important thing is to say Thank You but totally understand how life gets in the way and this can slip down the priority list!