There are a few different ways to express missing somebody.
Generally saying that you miss someone (hence, you want to see them). This is the most casual option available to you.
I miss you.
**I think this is your safest bet.
It's been awhile, I miss you!
This is a really strong missing; a yearning for something that you can almost feel. Maybe it's painful, maybe it's nostalgic.
I can't believe it's been a year since my grandpa died; I miss him so much.
After living abroad for four years, I've really begun to miss my mom's cooking.
This emphasizes the result of your missing them (you feel lonely). Not a native, but it feels relatively strong to me.
Please don't go - I'll miss you.
You're going home (back to your country) tomorrow? Man, I'll miss you!
After you (plan to) meet?
And then, once you arrange a time to meet or meet up again, you probably want to express that too, right?
I can't wait to see you!
I'm so glad to see you!
Next Thursday? Can't wait!