Here's the completed email with the verbs in the correct tense:
Dear Jean,
Hi! How are you? Tammy and I are in sunny Madrid now! We 1) arrived (arrive) three days ago. It's absolutely beautiful here and we 2) have already taken (already/take) lots of pictures. Yesterday, we 3) went (go) to a museum and then we 4) walked (walk) around the city. Spain is amazing! This is definitely the best holiday I 5) have ever had (ever/have)! What about you? 6) Have you decided (you/decide) where you are going on holiday yet?
Well, that's all for now. I'll see you in two weeks. Take care!
Love,
Charlotte