It seems that "might as well" has a few different meanings depending on the context.
For example, you have the "might as well" in the sense that you're already going to have to do something, so, it makes sense to take advantage of that opportunity and do something else. Like: Ah, we're going to pass that bakery on the way home, we might as well get some shuu cream while we're there. Or: I'm going to have to wait at the station, so, I might as well get a shuu cream at my favorite bakery across the street. Basically the sense is that you've already mostly completed the action, and because of that, you _might as well_ just do it. If that's how you mean it, you could say something like: どうせだから、食べましょう。or どうせ~なら、食べましょう。 Where ~ is what you where doing leading up to the shuu cream.
Another sense is that you went out of your way to do something, so, you should take advantage of that opportunity to do something else. Like: We drove three hours to get here, we might as well stop in at that famous bakery and get some shuu cream" in which case using "せっかく” might work. For example: 折角ここまで来たので、あの有名なお菓子屋さんのシュークリームを買って帰りましょう。
Or there's the sort of dejected, not really caring what happens, type of 'Might as well' -- which is probably the "might as well" that you're trying to express. In that case I think what mysticfive is saying is right on. Also something like: うん、いいんじゃない？ is probably also a good option if you're talking to close friends. If not, feigning some enthusiasm may be the best way to go. :)
