Echidnas are fairly straightforward. They are diurnal, so why they are sometimes put in a Nocturnal House is beyond me (except of course for the Longbeaked). However, they will sleep during the day, so if you have several in the one enclosure there is normally at least one wandering around to give the public something to look at. Often exhibited with koalas or small wallabies.
They like to dig, not burrows, they just like to dig straight down until all that shows is their spines. So you need a substrate they can dig into. Many exhibits have woodchip or mulch - they will go through this and into the soil, but at least the chips don't make the enclosure look quite so messy. Echidnas also like to shelter under logs and grass tussocks, or other sheltering plants like Lomandra.
Fences do to need to be high, but should be solid as echidnas can climb. They are not very agile at all, but if you give them a mesh wire fence at some point one will try to climb it.
And not that it's necessary to have a large body of water, but in Taronga's Wollemi exhibit I once saw an echidna wade into the stream and go for a swim.
As I said, fairly straightforward; most exhibits are fairly similar, and they are not too hard too maintain - the diet would be the hardest and messiest part.
And just as an aside - I found the public love hearing keeper talks about echidnas. Most Australians recognise an echidna, but know absolutely nothing about them.
Hix