。
How about going for a walk after dinner?
Literal
Dinner-after walk [to-に] go-out-[nominal-のは] how be-probably.
A suggestion pattern: [verb-の]はどうだろう = 'how would it be if we X,' literally 'how do [we think] about the act of X-ing?' The nominalizer の turns 出かける into a noun phrase the は can topicalize. どうだろう ('I wonder how / how would it be') softens the suggestion into a tentative proposal. Casual but thoughtful.