June in Níkaia, Greece, brings average daily highs of 32°C and overnight lows of 22°C in the 2016–2025 historical record. Allow room to shorten long outdoor plans when the day's forecast is hotter than you want. A schedule with easy breaks and less travelling between stops will give you more choices.
In the record, at least 1 mm of rain or snow falls on 4 of 30 days on average, a daily chance of 14%. Rain has been relatively infrequent in this record, which leaves more scope for an outdoor plan. Still keep an alternative for a wet day: a low monthly average cannot tell you when the rain will arrive.
The calculated day length, sunrise to sunset, averages about 15 hours. Sunshine averages 14 hours a day. The longer daylight window gives you more choice about when to be outside; it does not mean every part of the day will be sunny. Sunshine is a separate measure, and those hours need not arrive in one uninterrupted stretch.
Start with the places you most want to visit, then keep the order flexible enough to suit the weather. Check current opening hours and local transport before fixing long day trips. The climate record can help with pacing and packing, but it does not describe what is open during your stay.
July has an average daily high of 35°C, 3°C higher than June. Its historical rain-day chance is 5%, compared with 14% this month. If your dates are flexible, compare those trade-offs with the kind of days you want. Neither set of averages predicts the conditions of one particular holiday.