I want to write my boyfriend a surprise love letter but don't know what to say any ideas?

I'd probably write about how much I love him and that maybe could you go out for dinner or go to the cinema or maybe just have a nice day out

Sorry if this was no help

Tough one... It all depends on what type of girl you are and what is your current situation. Age, status in relationship, experience, type (cool, funny etc...). You just have to be yourself when your writing him a love letter. As long you love another and that is from your heart, it matters a lot to a love one. As to suprising your bf, you can ask to do something you guys have done b4, be sure to keep in mind that you should know your limit when your writing your letter coz going over the limit can also damage your relationship. So be smart and know what you are saying. Suprise him in any form you want to, but make sure it's reasonable mature enough for your age and everything that is going around between you two.
Hope I helped, your choice to follow my advice, but also not to listen to one advice but listen to what you think is right for you and your partner.