Need 1 more destination

Do you have specific interests? If we knew about them, it would be easier to help you. When are you travelling? I visited Stockholm last year and really enjoy this city. I would say there is plently to do for more than 4 day.
I visited Helsinki and took a day trip to Talllin the previous year and also recommend it. You can spend 3 days in Helsinki and see most the main sights and then on Day 4 go to Tallinn. Bon voyage!

Mid-20s male. Probably staying in a hostel. I'm really an open book. Sightseeing, venturing, drinking, etc.

I'd probably add Paris (4) after London and replace Oslo (not much excitement there) with Copenhagen (3).

Unless you have side trips in mind, I would cut Edinburgh to 2 nights. I would add at least one night to London. Again, unless side trips planned, I would cut Amsterdam to 2. I haven't been there, but I would also cut Oslo and substitute Paris, or Copenhagen for 2 if you want a Scandanavian stop. If you are saying that you have 4 nights in addition to what you have shown, then keep Edinburgh and Amsterdam, switch Oslo to Copenhagen, add at least a day to London, and add Paris.

Or, head north in Scotland to the Highlands. You're very city oriented. Do you want to take a break and check out the countryside? If you're traveling by train, head north. If you like the outdoors and want to just to kick back and enjoy the Highlands, you might want to stay in Toni's fave place, Aviemore. If you want a mix then stay in Inverness. No matter what, do explore beyond Edinburgh in your four days. There's an easy day trip to Stirling which has fantastic castle. You can do that on your own using the train. A tour might be to the Trossachs or down to the Borders. Pam

If you have not been, it has to be Paris.

Kris, you do understand that a three night stay is only two full days in a place right? I think you should add a night to London , come on, its worth three full days at least , there is so much to see and do..
I would also add Paris, and subtract Oslo( Oslo is very pricey !!)

Current schedule: Dublin (3 nights) Edinburgh (3 nights) London (3 nights) Amsterdam (3 nights) Oslo (3 nights) - Have friends there
Brussels (4 nights) Thought about Paris, but my reasoning for not going on this trip would be because it's such a major city that I could go at another time. Post London, I will be solo (aside from meeting friends in Oslo). I did think about these combinations for the last 4 nights: Copenhagen (2 nights) & Brussels (2 nights) Stockholm (2 nights) & Brussels (2 nights) Brussels is my cheapest flight back to America, so it looks like that will be the final stop. What I can be flexible with is how long to stay in Amsterdam/Oslo/??? (If desired)/Brussels. Advice please. Pat, Oslo is there to see friends that live in Norway (They were international students I met in college) and at that point, I would be solo. From the safety aspect, it makes sense to go to Oslo. Thoughts?

Kris sorry no thoughts , but I have travelled solo and I don't think you need to worry about safety aspects in any of your destinations in fact not in most in Europe period..

Hello Kris. I see that this trip is to Great Britain and Ireland and northern Europe. I recommend : Do not change any part in your first itinerary. For your four additional nights, travel in Norway. You could do the "Norway in a Nutshell" Fjord trip from Bergen to Oslo. Try to fly from Amsterdam to Bergen in Norway. And fly from Oslo to the U.S.A. Or, if traveling from Oslo to Walnut Creek would take a very long time, you might consider traveling in the opposite direction. Go from Walnut Creek to Oslo, and go to Dublin last, and fly from Dublin to the U.S.A.

Berlin?