1

xYosemite is not "around" xLas Vegas. It's a roughly 9 hour drive.

There are plenty of easy hikes at the Grand Canyon, on the rim. Same with Bryce. For these two, "difficult" means getting below the rim.
Zion is more varied.
Take a look at each park's website, and look at the trails under "things to do."

2

You can't see Yosemite and Zion/Bryce in 8 days during winter. They're too far apart, in opposite directions, and the most direct route to Yosemite is closed for the winter anyway.

You should consider the following:

Death Valley for an overnight or two, then backtrack to do the often-recommended loop around the national parks of S. Utah and N. Arizona (Bryce, Zion, Grand Canyon South Rim). Still takes you in opposite directions, but the distances are managable.

-OR-

Take a more lengthy and leisurely tour through southern Utah and northern Arizona, possibly including Monument Valley.

You can expect winter conditions at Bryce - hiking will mean snowshoeing. There are rentals available near the park. Conditions will be more moderate at other parks, although south rim GC is often snowy, and trails will almost certainly have some snow or ice on them. If you want to hike below the rim, you can purchase inexpensive traction devices for your shoes at the store inside the park. The upside is that these parks are at their most beautiful and least crowded in the dead of winter.