Je m'excuse de ne pas avoir écrit plus tot. (I am sorry for not writing sooner)
J'irai á la plage chaque jours. (I will go to the beach every day) [This is good for bringing in future tense instead of saying Je vais aller]
Dis bonjour a ta famille de me part. (Say hello to your family on my part/ for me)
Passé mon salutations a ta famille. (Give my regards to your family)
Idk these are some quick ones I know off the top of my head for the letter, the one for the beach is good for a postcard too
Welcome! It's not much but it'll help with content marks. Also another tip for letters is say an opinion at the end of each point you make.
So say if one of the points was tell the penpal what you will do this weekend, this weekend is future so you would say what you are doing and then say something like 'ce sera chouette' (It will be brilliant)
If you were talking about food in a paragraph and it was not nice food, you could say 'c'etait degoutant' (It was disgusting)
That last one is good for bringing in conditionnel tense as well. It'll get you extra marks if you have used many tenses!