A lovely bouquet of red roses ....... or a rose to plant in the garden with an appropriate name "Ruby Wedding" or Congratulations - there are lots of David Austin special occasion ones in garden centres and they make a lovely long lasting present.
If you want a cake (quick and on the cheap). Tesco do a cake where you can have a photo put on them (edible). Take a nice photograph to the Bakery section in Tesco store, think only the larger ones do this service, takes about 10 mins to have done and it's quite a nice gesture. Think cost about £10.
You could also try and get some small-ish 'ruby' themed things - like a bottle of ruby port, a postcard of the ruby slippers etc.. the sillier the better really.
You could maybe out them all in a basket or hamper decorated with red/ruby tissue paper or ribbon so it;s kind of like a ruby pick 'n' mix.
We did this for my parents (as well as getting them a more traditional gift) and it was fun!
very much ruby coloured things and along the lines that wease suggested .....my grandmother even got given ruby lingerie which had the oldboy puffing - but things like cranberry glass work well, especially antiques if they like collecting.....
For my outlaws I did a pen drawing of their house and had it framed with a ruby border.....
Ummmm - for our own ruby wedding I made some bookmarks on Word with Hubby and my photo on and an appropriate little verse and thanking everyone for sharing our day (we had a party) I then laminated them and put some thin red satin ribbon through a hole at bottom. I gave them out on the tables at the party and everyone said it was a nice little keepsake. You could make just two for OH's Mum and Dad with their pics on and a little verse and the date, they would treasure them I'm sure.