go to deviant art and but a print from there. Alternatively download fraxel software and create your own masterpiece get it printed off...hey presto sentimental and materalisitc!
Or how about theatre tickets? or a gig or paint balling or rolling down a hill in one of them big blow up ball things.
Or London eye or a night in a hotel ;o ) if you're on a budget try late rooms I think it is they are usually pretty reasonable.
Or a watch?
I'd get something you can do together then you can take pics and you'll both remember it.
for my BF birthday this year I got rolling stones tickets, night in a hotel and dinner plus tickets to see black rebel motor cycle club in Nov a Kings of leon CD and speakers/radio alam for his Ipod..as you can tell he's big into music!!!
Happy birthday for whenever it is! and dont stress over it what ever it is will be perfect cos you got it for him and thats all that matters.