Fleece tents clipped to the side of the cage are the best hideys, in my opinion.
The piggies love them. We have a long piece of fleece and since our Badger tries to keep Tufty out of his space (growling and strutting), we prop up a box that a book came in, to make a front and a back tent, and then they both sleep in there peacefully. My husband calls it a "semi-detached tent".
I also like those little plastic footstools people are using--they have entrances on all 4 sides, perfect for piggies to keep a look-out. I don't have those because two would take up too much room. The fleece tents are great because you just flip them up to scoop out the poops.
I also have a loft made out of a fleece-covered mesh grid placed on 4 individually wrapped toilet paper rolls (with the TP still on! The kind of TP you get at Costco in huge blocks of rolls). Luckily my piggies do not chew these--and they are the perfect size 'pillars' for a loft. One piggie sleeps on top of the loft, the other under it. Like the step stools, there are 4 entries to the space under the loft. TP rolls are only about 4" high so the piggies slink to get under the loft and then sprawl out cutely. Often there is only one little paw sticking out but if you look under you can see a little nose.