For instance, Antipas did indeed have thousands of units of arms stored up, as Agrippa declared, but it may or may not have been part of a plot with the Parthians. On the other hand, it's extremely doubtful the cache of arms had anything to do with this betrothal to Herodias. Rome already had four Legions in Syria. (For a recent and fuller discussion about this, see my post on Herod Antipas' Army, from earlier this year.)
Likewise, Antipas most likely did NOT plot with Sejanus against Tiberius, even though Agrippa convinced the unstable Caligula this was true. However, there may be another seed of truth to Agrippa's lies. Antipas and Sejanus must have consulted on two things together, at least. Aside from an official release from his marriage treaty with Nabatea, Antipas got one other thing from his voyage to Rome in AD 27/28. He won the right to mint coins.
The earliest coins of Antipas date to AD 29. Though the city of Tiberias seems to have minted local coins from 23; see Madden & Hoehner for the details. I disagree with Hoehner, however, that this dates Rome's granting of official permission for Antipas to that same year. Coins in 29 as - if not more - likely mean permission came in 28.
Logically, if Antipas comes back from Italy in late summer or mid autumn, he must then have taken the rest of the year to obtain ore and commission the castings. Even commandeering Tiberias' local mint, assuming minor modifications, and even if the first coins were proofed by year's end, it may have been most logical to post date them a few weeks for a first time roll out. Thus, coins officially dated to 29 don't necessarily mean the right to coin was secured in 29. More likely, it was 28.*
The coins suggest at least one part of what "business" Herod Antipas must have "transacted" during his one known voyage to Rome of those years (Antiquities 18.111). Juxtaposing this voyage with the marriage contract to Herodias is what dates the trip to the year before John the Baptist's arrest - the arrest being generally 28, 29 or 30 in almost all accounts, anyway.
(Full disclosure, I have other reasons for thinking John the Baptist was arrested in 29, just as Hoehner had his own reasons for placing that event in 30. Tit for tat.)