Why would I use sex to say I'm sorry? I just apologise.
Nice dinner, doing a couple of chores that are normally his, picking him up something I know he'd like such as a magazine, breakfast in bed, putting aside stuff I had planned to spend time with him, watching a DVD that he's chosen, running him a bath, playing football, having a laugh, enjoying being with him...there's loads.
But I do it because I want to, not because I want to apologise. If I wanted to apologise I would tell him straight, possibly do him a little card or something and put it on his pillow, arrange his dinner to say sorry (we're quite juvenile sometimes!) but it depends what it is you want to apologise for. If it's a biggie just sit him down and tell him. Your sincerity will mean far more than a token gesture.