Can you suggest some simple UI assignment that can be given to fresher recruitment?

Why not just give them an assignment that links with a current project?

That way they have a real world scenario to work to so they will have actual restrictions that will force them to learn new things, experiment etc.

Then when they have done it, if you feel it isn't up to standard you can get an existing dev to develop it and then show them where the differences are, why things were implemented a certain way etc. so they can learn even more. But the bonus of doing it this way is if their work is up to standard you can add it to the project and they get the accomplishment of doing something meaningful for the project plus save a couple of developer hours along the way!

There must be some "easy" parts of a current project you could let them tackle (and I put easy in brackets as most of the time the complexity comes from the size of an assignment so I would suggest a small assignment rather than an easy assignment as yet again, the more you can stretch somebody without overwhelming them the faster they will learn)