It depends on how much you want to spend... they must be pretty special friends to have thrown you a birthday party! My friend threw me a surprise 18th birthday party and I got him a personalised bottle of wine (that was �10) and an engraved pen knife (�19.99). He's kept the bottle of wine because he thought it was so nice and has the pen knife's attached to his car keys. I got mine from
www.winegifts4u.co.uk. They do personalised teddy's too, which might be a nice idea for girls. Or, you could organise a trip out with them to a zoo or theme park maybe? Sometimes it's nice to do things with people to say thank-you...!