For a day trips on budget to places like Graz and Linz probably the best option can be Einfach-Raus-Ticket (more info -
http://europetrainsguide.com/200907166/Countries/Austria/Austria-OEBB.html). You pay only 28 euro for both of you and travel as much as you like on whole OEBB network (but you need to avoid IC, EC, Railjet and ICE trains)
Other option for journey to Graz is to purchase Spar tickets from Vienna to/from Slovenia (from 29 euro), but get off/board the train earlier in Graz. In such case you will be eligible to use faster IC trains. But even such trick will not save you a lot - regular fare Vienna-Graz is 33.7 euro vs. 29 euro for the Spar ticket.
So truly big bargain is only Einfach-Raus-Ticket, but be ready to spend more time in trains - regional trains aren't that fast and you will need to make some changes as well.
Same story with Salzburg and Hallstat - either Einfach-Raus-Ticket or purchase in advance international Spar discount ticket (for instance Vienna-Munich or Vienna-Zurich for 29 euro) and get off the train earlier.