First off, how much are you looking to spend? I usually buy hobby boxes which can be anywhere from $40 for a low end box all the way up to around $400 to $500 for a really high end box. If you're looking to buy something which has a few "hits" (autograph or relic cards) at the lower end then I would go with maybe some of the Panini products such as Threads or Rookies & Stars for football and maybe some of the Topps products for baseball like Lineage, Allen & Ginter (this has different cards as well as baseball in it), maybe Bowman Chrome or Bowman Draft Picks. In baseball it really all depends if you want to pull mainly rookies and prospects (Bowman Chrome for that) or star players. I like to buy some boxes for myself and my nephew and we open them together always looking for that big card. We opened up a box of 2008 Upper Deck SPX baseball a couple of years ago and pulled a Derek Jeter Jersey Auto #'d to 20 and he was so happy as well as I.
I buy a lot of my cards at a local show here in Michigan but I also buy from Blowout Cards online. They usually have the cheapest prices. If you have any other questions just let me know.