The SMK 900 is very light.
Open sights with scope rail.
Single and multi shot.

Cheap and great for youngsters/ oldsters.

The Smk P15 bullpup is also good and light.

The Brocock Bantam Hilite is a good choice if you want to spend more.

I have the 900 and the Brocock and enjoy them both.
Especially enjoy picking them up!