Let's eat out tonight. / Let's eat outside tonight. NO - Eat out = at a restaurant ... Eat outside = not inside a building (You can eat at home but eat outside. That's not an amicable behavior. / That's not a friendly behavior. NO - we just wouldn't say the first one. Be careful with the people you are making friends. / Be careful of the people you are making friends. NO NEITHER - Be careful of the people you make friends with is the closest phrase. We have to get it in our contract. / We have to put it in our contract. NO The first is about achieving something in a negotiation and the second is about the details of a contract. The first I am not sure to be able to achieve. The second I am; it is more an admin consideration.

"Let's eat out tonight." means you are suggesting we eat at a restaurant--not at home--tonight. "Let's eat outside tonight." means you are suggesting we eat outdoors, not inside a building. This could be at home or at a restaurant. "That's not amicable behavior." means the same as "That's not friendly behavior." (do not use "an" or "a" here) I would just say, "Be careful who you make friends with." but it is probably better to say, "Be careful with whom you make friends." or even better "Be careful who you befriend." Both your contract sentences are correct and equally good but I would just say, "It must be in our contract." or "It must be stipulated in our contract."

Let's eat out tonight = Let's eat at a restaurant. Let's eat outdoors = Let's eat outside tonight. Outdoors meaning not in a building, house or inside restaurant. They are not quite the same thing.