I've done a few 'majority' solo days at Universal recently. I was there with family, but several days, we'd split up right after getting there, and I'd not see them again all day.
I had some great times at Universal. I did severalrides using the SR queue: MiB several times in a row, Mummy, and Gringotts. Spent a lot of time hanging out in Diagon Alley, eating ice cream from Florian's, or enjoying a Butterbeer or Dragon's scale, and people watching. There's a great place by the Menagerie store, and several benches in Knockturn Alley. I admit, I don't really like eating alone at table service places. I am ok getting something from QS, and just eating on a bench in the shade, if I can find one. Curb will do too. A crepe from the crepe stand, or a jacket potato good for that. I've also enjoyed Today Cafe quite a bit. Checking out the shows would be good, Bourne etc. or the street shows, and maybe even some character meets. The TMs there were very nice to take pictures w/ my phone for me.
In IoA, enjoyed getting a Hogshead brew and/or Butter beer and just sitting in the area, people watching. Enjoyed tots from Green Eggs and Ham, and Fire-eater's grill. Spider man has a single rider queue. I think Hulk does too. I've not done the SR at Hagrides or VC, as I hear they don't move fast, but I would probably try and ride them in the regular line, if I had time, and wait times weren't bad, I also took my time walking the queue at Forbidden Journey, (before using chicken exit, I just can't do that ride anymore!)
Enjoyed LOTS of time for browsing/shopping, taking in the Easter eggs, etc.
Have a wonderful trip!!