I have found too much time lying down while awake anywhere causes problems. I think it is the lack of exercise, so alternating walking and lying may be just as good or better than sitting for a long time. A tablet is good for web browsing or looking over a text document while allowing frequent position changes, and okay for small amounts of typing while sitting, or tiny amounts while lying or standing. I haven't tried a good set up with a keyboard stand for lying down, but a laptop by itself was pretty uncomfortable.

Typed while sitting on the couch.

I've found it difficult to have the room to get into a position where my hands aren't resting on a laptop keyboard on a couch without using a laptop desk. A mattress gives me more room to spread out and shuffle around.