They don't need skyscrapers or other big city stuff. Just more 3/4 story buildings that use their available lot well. You don't even have to mandate that; just get rid of the restrictions and market forces would gradually drive things in that way.
A lot of the population of Boulder doesn't even get to live areas that are especially walkable. Gunbarrel and all the neighborhoods East of 30th sort of require a car. Technically you can still bike but downtown or the campus are a long way, especially in snow or rain. And, unless it has changed, the bus doesn't run all night...
You go around Boulder and it doesn't feel like a city — this is part of the charm and also part of the problem.