What Works Well
Daytime temperatures are warm enough for walking between sights without needing to constantly break for shelter.
Long daylight gives you more room to fit in Pałac Schaffgotschów, Arsenał Miejski, and Bastion Ceglarski.
The city balance works well, with both outdoor landmarks and Muzeum Narodowe as a solid indoor backup.
What Needs Planning
Medium crowds mean the main sights are not quiet, so you should expect some company at the most popular stops.
Prices are also at a medium level, so June is not the month for bargain-first planning.
Rainfall is not heavy, but enough rainy days can still interrupt an all-outdoor plan.