My idea is almost the same as Rowanwitch's yet totally different!
I also think that you should 'sell' the ring to your boyfriend in a formal transaction, so that he can then give it to you as a sign of your engagement (and, of course, of your love for each other). However I'd suggest that you should hold an 'almost-engaged' party where you call for everyone's attention and then offer him the ring for the price of a kiss. After he's given you a public snog (probably to cheers from your friends and family!), you can hand him the ring. It will then be his turn to go down on bended knee and formally propose to you ;-)
Chris