We recently shared an accessibility review of the National Gallery in Prague. There is also a very pleasant Café One in the building.
If you decide to come here, or if someone like in our case invites you, don't get scared by opening the door with many stairs behind. Just go back and turn a little to the right ... Wheelchair users get to the café through the main entrance to the National Gallery, where there is a platform that is serviced by gallery staff.
The table was quite good to get under the table with your wheelchair and there´s enough of space around. You don't have to worry about the wheelchair getting in the way of a waiter.
In addition to good coffee and many kinds of teas, you can taste soups, sandwiches and other light snacks.
I wish you a pleasant break there and after refreshment definitely go to the National Gallery, really worth it...