Both sentences are natural. Wayne gives a good explanation of 'might as well'. 'might as well' is a little unenthusiastic. ie. You'll do it, but you're indicating you're not particularly excited or looking forward to doing it. 'could' is a good alternative if you don't want to show this reluctance or lack of enthusiasm in taking her.

"Might as well" usually indicates that there's some reason not to do it, or that you're giving in to pressure, or that there's no better alternative. For example: She said she has been there many times, but really wants to go again. I don't see the point, but if she really wants to I suppose we might as well take her there again.

That's good.