What would be an attractive way to display a magnet collection?

If you get a nice and big picture frame, you can arrage all the magnets inside and display them in that. It looks pretty cool!

https://www.pinterest.com/gracevdavid/travel-magnets/

I've seen old baking pans hung on the wall to display magnets. Hit your thrift stores or garage sales if you don't have one already. Take a magnet with you to be sure it sticks.

You can make a shadowbox with them and hang it on the wall!

There is now paint available that will allow you to attach magnets to it! Rustoleum makes it. Apparently you can paint any size you need on a wall. I haven't tried it yet, but it sounds interesting.

i have a sheet of steal from a big box store, you could cut it to size, frame it, and put you magnet collection in it

You can purchase the metal holders for knifes. Put them on the kitchen wall and place magnets and knifes on the strip.

We got some of those antique desk drawer things that have lots of different size little "shelves" - it's a printer desk drawer. We put in alot of stuff like magnets and do dads and stuff that I always loved. Many miniature things. Turned out to be a good use for the drawers.

I use drain pans from AC units the old used ones have interesting rust stain patterns

Make a magnetic board any size and shape just paint the board with magnetic paint and it is now customized.

Decorate a large cookie sheet.

You could put them on metal pencil holders, canisters that are metal.