Various ways of saying OK?
Here are loose translations: 

好的 = alright!

好吧 = okay, fine... (Kind of like... going along with it)

好啊 = sure! (As Wendy said... a bit more of an upbeat tone)

好 = Okay.

行 = Sure. I guess that works

恩　= Colloquial form of grunting in agreement... kind of like a verbal nod of approval

可以/可以啊 = I can/Sure!

Or, if you agree with what someone said... You can repeat the verb, or similar verb (in this case replace 去 (go) with 走　(walk)):

A: 我们去公园吧！ 
B: 走啊！