Best toys for making dogs think?

What are the best toys that will keep a food orientated dog amused for ages? I have a kong for each of them which are great, also we have a dog brick which is good but I need to start putting sticks and things in between the blocks as they have it well sussed now! A bone is always good too. Can anyone recommend any other toys which aren't too expensive, along the lines of those mentioned above?

We had a couple of those big plastic balls with a little hole in them that you put biscuits in. They spent hours rolling them round the room waiting for the biscuits to fall out - that is til Dennis (rottie) discovered he could just chew through the solid plastic to get to the biscuits

If your dogs dont swallow things they shouldnt then you can make your own
A plastic drinks bottle with some food in it - make it harder by adding some cheeze and nuke for 30s - then really let cool down - putting the lid on too makes it harder

A loo roll tube folded over with treats in

Hide treats for them to sniff out

Clicker training tricks is the best mental stimulation - 5 mins of that and they should zonk out

We have 2 of those Nina Ottenson doggy board games that you hide treats in and the dog has to work out how to get them. Unfortunately this is the reason why Chery and Parker have put on a kilo each I think! I am now putting them on sale on my e-bay site! They did enjoy them though.