Find the toy. Let the dog sniff a toy and then hide it somewhere. Guide the dog around the room (think Police dog handler) and ask the dog to 'sit' when he finds the toy. Praise the dog and throw the toy as a reward. They pick up on this game almost instantly. You can extend this game from finding a hidden toy to finding any hidden item.
Hey Kaznalf, I'm from London where are you? To keep my girl active I use a kong and hide some of her treats in the garden. Although last night she was eating something and thought it was poo, told her off and the realised it was her treat :felt really bad :-[
You can invent your own variations on those, too. For instance, I used to "hide" duck wings in easy to find places and then make them progressively harder as my Bess got progressively better at finding them.
Puff [YF, AKC field line (from competing HT/FT breeder) 62 lbs, dob: 8-'01]
One other suggestion ~ I don't know if you have younger children, but my daughter (12) plays "Hide and Seek" with Shelby a lot. My daughter will put Shelby in a sit/stay, and then hide. When she says "Okay", Shelby knows to look for her. Besides working on the sit/stay command, it's hugely entertaining to watch!