i wouldn't go for the personalised champagne, it could be horrible stuff.
i would either go for a good bottle of champagne (likely to set you back about £40 e.g. laurent perrier rose)
or a plant for the garden is a lovely idea. We received some lovely rose bushes for our dd's christenings with names that meant something (Irish Eyes, Sophie's Rose, champagne moments) etc..
or if you have a nice photo of them get it framed. it's hard to get nice pics of both of you.