There's no perfect trip — only your trip. Travel is one of the best ways to learn about yourself, but only if you're honest about what you actually want. Don't let other people's ideas of a 'perfect' experience cloud what matters to you. Plan the trip you want, not the one you think you're supposed to want.
Behave yourself. If it's illegal or inappropriate at home, it's no more acceptable just because you're in another country. The country you are in has laws that you are subject to. Don't forget it.
Experiences over views. We live in a beautiful world, and you will see amazing things while you travel. That being said, the more you can interact with people and the culture of the country you visit, the better time you will have. I hardly remember my trip to the Trevi fountain but I remember every moment of the solo flamenco class I took. A hands-on cultural experience is usually more meaningful than a quick stop at a tourist landmark.
You don't always need an 'authentic' experience. In the travel community, there is a push for a more 'authentic' experience as opposed to a touristy one. This stems from an elitist fantasy many travelers have that wherever they go, they can integrate with the locals better than their first-world counterparts. When planning a trip, try to ignore the travelers telling you to be more authentic. Unless you completely speak the language and are willing to move there long term, you will not integrate on your three-day trip to Singapore. That's ok. There's no shame in visiting popular sites or buying the things that help you feel grounded..
Study up. Some travelers avoid it, but I strongly recommend learning a bit of local history and language before you go. Before going somewhere, learn a little about the local history and language. There is a stark contrast in the interactions you will have when speaking to a local in their own language (even badly) as opposed to assuming they speak English. In some places, like parts of Eastern Europe or East Asia, you'll often find excellent English speakers — but don't count on it everywhere. If you can, before you go, read a book on where you're going or listen to a podcast or two. I promise you, historical context before the trip will make your visit much more interesting.
Be honest about what you can do. Not every place is a good destination for everyone. There are places that I would go as a young, able-bodied person that I would not take my elderly grandmother to. When you pick a destination, consider your own needs and limitations: Will I be able to communicate with them? Are there accessibility needs I have that might not be available here? Is this place safe? Some places are best left for experts and don't lend themselves to tourism.
Talk to strangers. Hostels, walking tours, and tourist sites are great places to meet strangers. They are almost always interesting and friendly. Be smart: avoid sharing your lodging details, and it's usually better not to mention you're traveling alone. But a conversation with a stranger will be nearly always be an interesting and rewarding experience. I have learned more from a ten-minute conversation with a foreigner than I did in two hours in a museum.
Museums and cathedrals will all start to feel the same after a while. I almost always avoid them now. Unless you have a strong personal interest in history or religion, you might find that many museums and cathedrals start to blur together. It's okay to skip them.