There is stinks:
v. (intransitive, informal) To be greatly inferior; to perform badly.
This is comparable to the definition of sucks:
v. (intransitive, slang) To be inferior or objectionable: a general term of disparagement, sometimes used with at to indicate a particular area of deficiency.
v. (intransitive, slang) To be very undesirable (see also suck).
Instead of saying "this sucks", one could say "shucks!" or "nuts!" This would depend on how much something "sucks", of course.
(colloquial) Exclamatory response to a minor disappointment.
Indicates annoyance, anger, or disappointment.
To describe the sex act, you could say fellatio.
Finally, you can opt for sarcasm: "that's interesting" instead of "that sucks". This relies on delivery and how well your audience knows you, otherwise your words may be taken at face value.