Suggestions for early morning arrival

Wander the Ramblas down to the sea and back. There are hop on hop off buses; one ticket is good for the entire system. You may not need a dinner reservation if you dine early. Check your tour info to see what you won't be seeing during the tour. You can visit a place not on the tour.

I don't know if you'll find much in the way of dinner, which is generally much later in Spain than elsewhere (9-10 p.m.), but there are plenty of tapas bars where you can fill up.

Head down the Ramblas from Placa Catalunya to Colon, passing La Boqueria, Gran Liceu, Placa Real, Palau Guell.
Turn left and head along the waterfront towards Barceloneta - do not head over to Maremagnum (lots of shops).
Either, head to the beachfront promenade, then metro return to the hotel from Barceloneta to Urqinaona.
Or, wander back through La Ribera via Basilica de Santa Maria del Mar and the Cathedral.

Thanks, I will route this out on my map. I need to consider what will be seen with the tour, which meets on the Monday, and what will be open/closed on Sunday/Monday. Will welcome suggestions for those days also, as I understand and some places are closed. Getting my research done....

We all have different approaches but ours is to take a nap - max 2 hrs. - in mid afternoon. Recharges our batteries enough to have a pleasant, evening dinner. We learned that approach after years of trying to gut it out and stay awake for an early dinner. Not always but the majority of the time we find that our room is ready when we arrive for check-in. Barcelona is big on tapas bars. You may find that a good substitute for a heavy evening meal prior to retiring and those bars are almost always available.

Yes I would try to get into your hotel room by 3pm and nap til 6pm.
Another walk, some drinks and dinner at the usual Spanish time of 9pm.
Hopefully this will minimise the effects of jet lag in ensuing days.
How many days do you have to fill before the tour begins?
I wouldn't explore anything in depth on this opening day if you have plenty of time, just get your bearings.

Don't be so sure that you won't be able to check in when you arrive. Twice, when I have arrived in Madrid about an hour earlier than check-in time, my room was ready and they went ahead and let me check in. If you are lucky, you might get right in and be able to shower before heading back out. Don't worry about dinner. You have no trouble finding somewhere to eat, even early. You could walk through La Bouqueria (not sure I spelled that right), a huge fresh food market on the Ramblas and pick up fruit, juice, and lots of other goodies. And there are always places like KFC and McDonald's, if nothing else.

I was in Barcelona with an early morning arrival in September. I notified my hotel prior to arrival that I might need to store my bag at their front desk. When I got to the desk (a short walk from Placa Catalunya) they let me check into my room. It pays to notify them ahead of time you are coming early. After I unpacked quickly, I headed out to the Barri Gotic to follow the Rick Steves' Spain book recommended walk through the old quarter and it was marvelous. I tried to avoid the tourist crush of Las Ramblas until I had had some sleep. Also, check out what types of concerts are offered at Palau de la Musica Catalana. It is beautiful and not far from Placa Catalunya. Have a great trip.