My two (DS, 6, and DD, 3) are really into the cars and they have several as does my nephew. My SIL has got my nephew the Mack truck for his birthday. I think it is about £15. Also my DS has the Cars playstation game which he loves - don't know if they have a Nintendo DS version.
Easy party suggestions:
bubble machine (if weather OK)
jumping on bubble wrap (has to be big bubbled variety) - great hit usually
order helium balloons (pre-filled) on line - doubles up as decorations and can be present instead of party bag if offered with a piece of cake (get colours both boys and girls like)
Party Pieces has good pre-wrapped pass the parcels and pin tail on donkey etc.