Our sons bought us a couple of nights in a lovely guest house near where we went on honeymoon Original hotel no longer there! Would have also been happy with theatre tickets as well!
You know them better than we do (obviously!), how much is the budget?
No need to answer, a taxi to a restaurant for a pre-theatre meal, then the theatre, then a taxi home where family will be gathered with a cake?
They are going for a meal on the actual day, to the very nice michelin starred restaurant near where we live... i was going to offer to pay for that, but as they were going to go anyway it seems a waste. The theatre will be something extra and unexpected i suppose.
My dad is not much of a theatre person, although they really enjoyed Jersey Boys and The Ratpack. The only one i think he might enjoy is Dreamboats and Petticoats?
My sister and I took my parents out for a meal and then surprised them with a booking at a plush hotel in town for a couple of nights later - we printed off the details then made it into a wee scroll with a ribbon round it to present to them at the meal. And I secretly arranged with my mum's boss for her to get time off work so they could go. They loved it.