Seconded on the Chromebook. Another option is a secondhand netbook, like an Acer Aspire One - very small and easy to throw into a bag, very cheap since they nev

Seconded on the Chromebook. Another option is a secondhand netbook, like an Acer Aspire One - very small and easy to throw into a bag, very cheap since they never really caught on and vry good battery life. You might have to relax your HiDPI requirement for a cheap machine though - you're not going to get a fantastic screen AND a low price.