1) As a class, brainstorm a list of general, positive statements to make. For example:
It's such a beautiful day.
Wow, it's nice and sunny today.
This food is delicious.
2) Once you have a nice big list on the board, put students into pairs. Have them take turns making positive statements. But when one makes a positive statement, the other one will say something negative that takes away from the positive statement. For example:
A) Wow, it's nice and sunny today.
B) Hmm. We'll probably get sunburned.
A) This food is delicious.
B) But it's way too expensive.