Everyone wants tips on cheap travel. There's a few ways to go about it, sometimes it works sometimes it doesn't. But I just search search search.
1. Be flexible on the dates and destination. It is usually cheaper to fly in the "off-season". But, if you are dead set on going over summer be flexible with the destination. Google flights has a nice map feature where you can look at most major airports and determine what city or country you can fly to, for the cheapest. (I also have a subscription to scottscheapflights.com.) It is usually fairly easy to fly country to country in Europe or even add the experience of taking the train. For example, I was dead set on going to Ireland this summer, but flights were $800+ at the time. I then found a flight for $600 to Denmark and decided thats where im starting, then flew to Ireland for $40 to continue on with my trip. Yes eventually you have to make your way back to Denmark for your return flight, but if you want to do multiple countries on a trip, the difference in price isn't notable.
2. Find a major international airport that you can fly out of. Some airports are horrible for international deals and travel. I'm based out of Phoenix, and I rarely search out of PHX airport. Try searching LA, Las Vegas, Denver, Chicago, and New york. Doing this, helps reduce the amount of flights the airlines has to combine for you. Less convenient for you, but also can be cheaper in the long run.
Example: Found a flight 10:30 pm Chicago to Copenhagen. Next I have to search for a flight PHX to Chicago that gets me there by 7 pm, that way I have time to "check out of Chicago", get my bags, find the international terminal and re-check in. Again, less convenient, but comparing it to PHX- Copenhagen, I saved well over $600.
3. So I've booked a flight out of Chicago, but now I have to buy a flight to Chicago from my home city for an other $150-$200? Not quite. This is where airline points come in handy. Every chance I get, I use my Southwest Credit card and rack points up all year long, that way I have a FREE flight to that city. If you don't have an airline card, find one you think you would like, I recommend Southwest and United, they all tend to have great sign up deals where you spend a certain amount of money in 3 months, and you get roughly 40,000 - 60,000 points. If you play your cards right that could amount to 3 or 4 single way tickets. My SouthWest Card referral link.
4. For stays, I recommend Booking.com. Sometimes I venture over to Hotels.com. Both have there perks, but my preference is Booking.com. You level up with them, you can achieve free upgrades, free breakfast and sometimes they pay around $10 or so of your booking.
I tend to stay in the city center of where I am traveling to, that way I am close to what I want to see or do, and the subway or train is close by as well. I like to keep my options open. I avoid Hiltons or Marriotts, I look at small local hotels, read reviews, and go with my gut. Reviews tend to be accurate and I have yet to book a hotel, show up and hate it. Overall, my stays over the years have been what I have expected. If you are booking through Booking.com be sure to use my affiliate link.