Formal:
* Je pense que oui. (This is the most literal translation and is quite formal.)
* Je crois que oui. (This is another formal option, similar to "I believe so.")
Informal:
* Je pense bien. (This is a more casual way to say "I think so.")
* Je suppose que oui. (This translates to "I suppose so" and is more neutral.)
* Oui, peut-être. (This means "Yes, maybe," and is a bit more hesitant.)
* Je crois. (This is a very short and informal way to say "I think so.")
You can also use a simple "Oui" (Yes) in informal situations, if the context makes it clear that you are agreeing.
The best choice will depend on the specific situation and the tone you want to convey.