I did indeed travel solo ...... no problem ..... everyone was very friendly, both couples and singles. Going out to meals alone, which bothers some folks, didn't happen to me. The included meals were very generous and when by myself I'd just get a sandwich or empanada. More than enough! and then there was that delicious ice cream.
I would have loved to go to Bilbao, but time didn't permit. Those that did go thought it was great.
I didn't go on the "Manises/Lake Albufera" optional because I wanted to spend more time in Valencia seeing the cathedral and the ceramics museum (Google it ..... it's small but fun .... in an old mansion), and enjoying the cathedral square. Everyone who did go on the optional seemed to like it very much.
An interesting note: more than half our group were Inner Circle and one lady had taken more than twenty trips with OAT/GCT.
Tipping: many of us tipped in American dollars. Before I left I stocked up on $5.00 and $10.00. Euros, of course, would have been better, but nearly every day required several tips and it was hard to get the necessary denominations.