Whilst Engagement gifts are not compulsory, indeed no gift is, it is a lovely opportunity to celebrate the happy couple's commitment and promise to wed. With weddings often taking upwards of a year or more to plan it may be a while before you bestow a wedding gift to your friends. One of my favourite Engagement gifts was a pair of champagne glasses - I can't help but feel a little decadent sipping some sparkling Rose from my 'posh' glasses. It can be tricky choosing an Engagement gift which doesn't preempt what you may give for a wedding. Gifts which aid in the planning of the wedding are likely to be well received, probably mostly by the bride (terribly stereotypical, but you know it's probably true).
Another idea is to give something that you can then build on for your wedding gift, a sort of 'taster' of what's to come. Perhaps a set of mugs, to be followed up with a coffee maker or other associated items.
And, when inspiration is lacking or you have a couple for whom it's difficult to buy for, a bottle of Champagne is a mighty fine way to celebrate - if you're lucky, they might even pop the cork whilst you're there and share. I did say might.