60 can be a significant Birthday. It was until recently the landmark for a woman's Retirement. On my Mothers 60th Birthday we Celebrated both occasions, it probably made it easier.We used it as an Event for a family gathering, including close friends. Held the Event at my home, as she does not like fuss, or being the centre of attention. So it was a gathering of people she was fond of, sharing food, refreshments ( no alcohol, as my Mum does not drink alcohol). Easy listening music, as background music, allowing her to chat to people. She had been retired for a few months, having been offered retirement / redundancy earlier that year. She was now taking her youngest grandson to nursery, as his mother had taken on employment. So she luckily could share fact that she enjoyed spending more time with her grandchildren. Having developed a closer bond with the youngest, he was delighted to spend time with his Gran. She collected him from nursery on some occasions, as his mum's working hours varied.
We chose to get Mum some practical items. She enjoyed the time she spent at the gathering of family and friends the most. Chat and humour being part of the day.
Maybe your Mother in Law would enjoy spending time with the family etc. You mention she likes gardening :- Maybe some garden furniture, a tree to mark this significant Birthday etc would be useful, practical gifts which she could appreciate. A fruit tree would also produce nourishment which she could enjoy and possibly share with others in the future.
Best Wishes. Hope you all enjoy the significant Birthday.Celebrate!