I think peoples interpretations of creativity in software development differs greatly, so its always a matter of opinion (some people see it at as having fun, others see it as having freedom to choose interesting/alternative technologies, others see at as having time/resources to experiment and innovate... etc. etc.)
I've worked in start-ups as well as large and small companies, each had the opportunities for creativity, but in different aspects of the development process - some allowed for creative under the hood solutions to tricky or repetitive problems but with very preconceived expectations with regards to UI (this is particularly so in local and central government) - others allowed for creative UI/UX opportunities, but had little love for the behind the scenes code being crafted (particularly common in the content-management heavy web space, where very time-constrained and competitive contracts are in place) and start-ups provide opportunities for Innovative solutions to business problems, but don't always require that much innovation when it comes to code (in fact many start-ups I've worked for suffer from trying to achieve too much technically before actually having a suitable business model / customer base ).
In all cases I've found the Morale of the entire team (not just the developers) and schedules with enough allowances for experimentation/creativity to work best. Both of these can to some degree be nurtured by the development team themselves.
I notice a number of people suggesting start-ups being a good environment for creativity - and I would agree with that statement, albeit with one caveat that often they aren't outcome focused / constrained enough, creating things without customers or dog-fooding the product your developing (if appropriate) can lead to an eventual creativity slump - as solving software problems creatively is often easier when there are some constraints in place as a guide (time, budget or technical).
I have seen some start-ups flounder just trying to select a suitable/interesting technology stack (or contemplating creating their own from the ground up) rather then focusing on actually solving the business problem they set out too.
Personally my favourite space to work in creatively is running a company and developing your own shrink wrapped products for sale, it provides a good mix of opportunities for creativity with some constraints (that I like because it helps me focus on developing an appropriate solution) - also I found creative and well crafted solutions normally have more merit when they apply to multiple customers.
My 2ø worth.