1. vienna to salzburg and back
Regular fare on this route is 47.5 Euro, but you can actually save quite a lot by purchasing discounted international tickets from Vienna to Munich (or Zurich) + de/borading the train earlier/later. In such case Vienna-Salzburg-Vienna can be done for 58 Euro only. You can as well use Einfach-Raus-Ticket (allows group up to 5 people to travel whole day on regional trains in Austria) - it costs 28 euro for the whole group (in your case - two passengers).
2. vienna to budapest and back
Discounted tickets on this route start only from 19 euro. Budapest-Vienna bought in Hungary starts from 13 euro.
3. day trip from vienna to wachau and krems
Regular fare on the route Vienna-Krems is 13.9 euro, but since there are two of you - purchase Einfach-Raus-Ticket for 28 euro (allows group up to 5 people to travel whole day on regional trains in Austria, Vienna-Krems is served by those)
As you can see: individual tickets + Einfach-Raus-Ticket are much cheaper than the pass, so forget about Rail Europe and their pass offers