I would like to end my trip on a relaxing note, on a warm beach (preferably 70-80F temperature). Can anyone suggest an area to go that may be a little closer than going down to Greece?
Baltic Sea can be really nice if the weather cooperates.
It is not unusual for air temps to hit 30 to 35 C (85-95 F) in northern Germany and Denmark during July and August, with water temps of the Baltic Sea hovering around 18-21 C (65 to 70 F). On the other hand you might also get wind and rain and air temps around 18-25 C (65-75 F), really hard to predict at this point. I would decide whether to go to the Baltic see or not when you are in Copenhagen and Berlin and can look at the 14-day forecast.
Since you are going in the general direction of Eastern Europe, I would also suggest you look at lakes.
In Czech Republic the most popular summer destination is the Lipno Lake, also nicknamed "the czech ocean". It is halfway between Prague and Munich not too far from Cesky Krumlov and the Bohemian/Bavarian Forest national parks.
In Austria at the border to Hungary you got the Neusiedler See. Because it is a relatively shallow lake water temps easily reach 30 C /85 F after a series of warm summer days, downside is that it also cools relatively quickly down to 21 C / 70 F after a cold rain.
Lake Balaton in Hungary is similar to the Neusiedler See, very shallow and easily heats up. Additionally Lake Balaton also has several hot springs, which makes it really appealing if you want warm water to soak in.
If after all this you are still looking for an alternative to Greece, consider Bulgaria and the Black Sea.