I'd second some kind of day out that your mum would enjoy. For example if she's a keen gardener maybe a local stately home with beautiful gardens followed by a posh lunch. Or maybe a trip to the theatre. Or a trip on steam train/orient express Or as mentioned posh afternoon tea.
Hi, thanks. I did ponder the afternoon tea thingy but I took her to the Ritz for a previous birthday so I probably can't top that one! I do like the idea of something like that though instead of a gift.