Cooking facilities - should be available, but might vary as to what there is exactly.
For food shopping - best to head for a main supermarket, such as Eroski Syp or Caprabo, the former is about 20 minutes walk, the latter a bit nearer. There is a "tourist supermarket" Colber close to the apartments; it's good but you will pay more.
Cheap eating places - don't know, depends what food you like. The Red Lion below Siesta 1 is quite reasonable. Also pretty good for family entertainment. There are a few places close by which are comparable - Yummy Yummy, Peach Pit, Cheers - and these all do food at reasonable prices, too.
In the Bellevue complex (the Siestas are more less a part of this although they are separately run), there is a fair amount of entertainment on offer. There is also a good kids'club set-up at the apartments.