I'm a "beginning" composer (not very good yet, but I know basic theory), and an orchestral song I'm writing will require modulation from Bb minor to a "related" major--in this case, Bb or Eb. I have almost a solid two minutes of music in Bb minor already, and I'd like the final modulation to Bb major to sound somewhat surprising. The problem is that I have no idea what key(s) to modulate to (if any) for the B section and beyond before the final modulation.
This is similar to the 'Tierce de Picardie', where a piece is in minor until the last cadence, where the sun comes out with a parallel major chord to finish. Generally, it's going to be a perfect cadence - listeners are going to expect the final tonic minor - so in your case, an F or F dominant seventh harmony leading straight to Bb major will suffice.
For a surprising effect without a Picardy third is to deceive: attempt to move away from the established B-flat minor, toward an unrelated key, smoothly, so the listener anticipates a new tonic. Then, at the last moment V7_I !
If you want 'surprising', perhaps pick an UN-related key to move to?
But if you want the 'sun coming out' effect, an excellet way to get from Bbm to Bb is 'just do it'. The more you contrive, the less the effect.