If you only need to show client capabilities to you can use our demo site, http://sense-demo.qlik.com
If you sign in with your account to the site you can also edit the demos in the browser.
Another option is to navigate to http://localhost:4848/hub with Qlik Sense Desktop running and you can still demo from the browser. The Desktop client and the server web client share most of the same features.
I don't think there is a demo version of the backend however.
Ninja edit: Of course you can also use the free cloud if you want to demo your own data, https://qlikcloud.com/