Our family of 3 (parents/teen) made a day of visiting the Arch, museum, and the river boat. It was nice to be out on the Mississippi, but I think if I had my time again, I'd do a cruise along a more picturesque part of the river. Perhaps the most interesting things we saw were the huge Mississippi river barges which are plentiful along this stretch of the river, whilst the only real photo opportunity was of the arch itself. The tour was pretty uneventful, with the guide and captain taking turns to narrate various points of interest, which included a few old riverside buildings now in disrepair, the various bridges we passed beneath (although the cruise doesn't make it as far as the Chain of Rocks Bridge unfortunately) and the Gateway Geyser, which was in full force as we set sail. The boats are docked pretty much right in front of the arch, so they were nice and easy to find and the boarding was very smooth, even if it was a bit of a bun-fight to bag the best seats. Having just been to the top of the Gateway Arch, we decided to take an hour-long sightseeing cruise along the mighty Mississippi.
