If you have to tell him what to buy, it's not a gift, it's just something else on your to do list/mental load for you to deal with, like you say. Plus if you go this way, in his mind, he got you a lovely gift for your birthday, when it's nothing of the sort.
But obviously, if it's anything you have to use, wear or look at, you probably do want to choose it yourself or else you'll likely have to put up with something that's not right. So I don't see the point of giving these sorts of things as gifts.
Which leaves asking him to get something small and thoughtful, that shows that he knows you and what you like. Gifts don't have to be expensive to be worthwhile and it's often a lot better if they're not, because then you don't have the worry of the money being wasted.
Ask for the vouchers to add to your stash and also make sure you take the time to make a day of the experience, with a nice lunch and either someone to go with or if you prefer, time to do it alone so you have time to browse in peace without having to think about anyone else's needs. Remember it's your birthday so you get to choose, don't have him say 'that's not a proper gift'. He can also get you a token consumable, whatever you like chocolates, cheese, coffee, tea, gin, whatever.