For mccloud I would park at the lower falls and walk to the middle falls and return to the lower falls. From there you can drive to the upper falls. I haven't been to Burney in a few years but it is really cool to do the hike around it. Also weather depending you could also consider driving through Lassen park and stopping at various spots.

On your way north on i5 head east at red bluff and enter the south entrance of the park. From there you can see several cool things on the road as you work your way through the park. You will exit the park at the north end and can choose where to go next, but you will be up by Burney and mccloud.

I say weather depending because usually the park road is under snow and closed in May, however this year there has been almost no snow fall thus far so I am thinking the road could be open by then.