I do diary's for all my under 5's and it's basically the same as everyone mentioned here with time I did nappy changes (wet or soiled - was told not to put 'dirty' or 'poo'
), bottles & food (time, what had and how much), naps (time and how long - how i.e. cot, in car etc), what we'd done, where we've gone, any achievements (i.e. baby sat up etc, mindee went to toilet on their own, mindee drew really good picture/practicing with pen and followed the dot to dot perfectly etc.), any problems i.e. baby upset more than normal/teething. Normally the older ones ends up being more of a summary of the day but for the babies, my layout is a bit like this (this is made up btw):
xxxx - nappy change (wet)
xxxx - bottle (left 1 oz)
xxxx - napped in car for xxx mins
xxxx - nappy change (soiled)
xxxx - lunch (ate half of main, little dessert)
xxxx - nap (in cot)
xxxx - woke up
xxxx - nappy change (wet)
xxxx - bottle (had all)
xxxx - dinner (had all)
Then after that I do a summary of the day like.........Today we went to baby/toddler group, they really enjoyed playing with the other babies. xxxx is doing much better on front, lifting head really high, sat up and played with toys by themselves, lots of rolling around on the floor etc etc. Then I might say either.....we've had a really smily and giggly day today with lots of babbling, very cute (and I quite often draw smily faces or winks) or we've had a lovely day but poor xxx has been suffering with teething today, poor love, which has put them off their food a bit. (that type of thing)
Probably gone too far there
but I wanted to give you an idea of what I put in mine 