One of the questions I have been asked by non-letter writers is, "What do you write about in letters?" My answer is almost anything and everything, although it depends on who it is you are writing to. Topics generally avoided are - Religion (faith can be personal, and may also be divisive), Politics (some politicians this year have said things I think are hurtful, racist, sexist or just plain silly), and Sex (yes please ;) - I actually have drafted this blog post on paper and drew the wink however my drawing looks evil). However, anything else is generally fine. Talk about roast turkey should be kept at a minimum with vegetarians, and should also avoid telling them about your child's dissection of a sheep's heart in science class at school (some brought in cooked or frozen hearts...). Make sure the letter is not all about you. This may be difficult for the first letter you send to a new penpal.
Speaking of days, it doesn't matter if you can't finish a letter in a single sitting/day. I tend to write the new date when I continue on a different day, even if it is just after midnight. It is a good idea, especially if you write internationally, the name of the month and not its number - a date last week was 10/12/2015. In the UK, this means 10th December 2015, however it can be interpreted as 12th October 2015.
You can talk about family - sometimes, they can be annoying. You can pick your friends, pick your nose but can't pick your family. Penpals can be great listeners - there's time to think and craft responses. Have we all, in speech, said something possibly hurtful without thinking? We may be able to offer advice and suggestions, or just provide a shoulder. It might not be good to keep things bottled up inside, however not every penpal would want to hear about your woes. There'll be close penpals, friendly penpals, acquaintances who are correspondents, akin to friendship in the 3D world (I didn't want to use "real world" because to me, penfriendship is still real friendship).
Some letters I've received have had me in fits of laughter. Laughter, after all, is the best medicine! You could admit to doing silly things - I once put sugar on my chips (fries) instead of into my cuppa (at least salt didn't go into the wrong place). Some people write diary letters but that's OK in moderation. What happens to you on a particular day can be of interest to your penpals but they don't need to know every detail of your day. The sometimes silly Five Questions.... challenge over on A World of Snail Mail can be cathartic, and maybe take you down memory lane. Now, which leg did I put in my trousers this morning?