I think a phone call to the attorney who was your primary contact at the firm (for me it was the Partner who interviewed me during OCI)followed up by a phone call to the recruiting director is the best policy. Speaking from experience, the folks who interview you feel somewhat invested in you and, though making a phone call is the most difficult for you, it will leave those folks with the most favorable impression of you. Down the line, if you want to work at a firm that you chose not to go to for your 2L summer, it will be to your advantage to have made those calls. People remember such gestures, especially folks in recruiting or attorneys on the recruiting committee. Good luck.