The cage sort of crates are the strongest (and fold down nicely) - and don't worry about them looking like cages, your pup doesn't know what a cage is!

I bought a black metal 24" crate on EBay from someone in my home town, so no postage, for just £10. It has two doors (I personally prefer 2 or even 3 doors) and a metal tray floor (the plastic tray floors are more prone to being chewed).
Alfie has loved it from day one. We had it in the kitchen, both doors open, nice cushion inside, treats on the cushion, from the time we brought him home; he found his own way in there, and out again, and in there and out again (I replenished the treats - which were just bits of his kibble) and then, of his own accord, flaked out in there a couple of hours later.
We have a black fleece blanket over the top and two sides in the day, and cover it completely at night.