Mortston near Blakeney makes a great day out. You only have a four-ish hour window of oppertunity (2 hours before and 2 hours after HW) but take a picnic and head over to Blakeney point. Head a bit further round the corner and you'll see lots of seals sun bathing on the beach. Google "Norfolk etc" and you'll find the details of the sea school there - use their postcode to find the slip and carpark.
Southwold makes a nice place to head for lunch. Launch at Yarmouth next to the lifeboat station or Lowestoft at Oulton Broad near "Pets Corner" (involves paying £10 or so to go through the lock to get out to sea but a decent slipway and loads of parking). Tie up and head to The Ship on the quayside for lunch. Alternatively there's a couple of resonable slips here. It's only a half hour potter North to Lowestoft where lunch and a mooring can be found at the Royal Norfolk & Suffolk YC.
Launching is also possible at Sea Palling off the beach - google Beach Rock leisure - they will launch you with their tractor for a few quid. From here you can head down to Yarmouth or Lowestoft.
Another favourite trip of mine was to launch on the opposite side of the river to The Ferry pub at Horning (slip is at the far end of the Broads Authority carpark for Cockshoot Broad, near Woodbastwick). From here you can potter up the river (at 4mph!) to Wroxham, or down the river to Ranworth. Both very pleasant days out and lots of nice pubs to stop at!
If you're new to RIBs I would recommend launching at Oulton Broad for your first outing - its really easy. The others are all a bit more tricky, especially the slip next to the lifeboat station in Yarmouth as the tide runs through there quite quickly.