Budgeting recommendations

My target budget is $150 USD per day.
Big American chains outside of the US (e.g., Marriott) are often more expensive than local chains, and more likely to charge ridiculous resort/destination fees, so I avoid them.
Europe and Asia are often better than North America at offering small (sometimes tiny) yet clean rooms at very good prices. I look for quality over quantity in the room; smaller rooms are also often cleaner. Traveling lighter unlocks a lot of savings: avoid airline baggage fees, open up public transportation to/from airports, and squeeze into tiny hotel rooms.
Scan through traveler photos of hotel rooms to audit for cleanliness. Bathrooms are usually a good indicator. Hotels with unwashable bedding tops are usually dirtier, unless you're in an extremely rural area.
The breakfast rate is never the best budget option.

Look for opportunities to build stopovers in tickets. Airlines sometimes offer stopovers at connection points, especially flag carriers at their hub airport, at little-to-no cost.
Avoid midday flying as that burns an entire day.
Plan how you are going to get to/from the airport. That 6am flight might be cheap but then requires a $100+ cab.
Low cost carriers can have a lot of hidden costs. More time to/from secondary airports and at bag drop lines can extend trips. Infrequent schedules make travel less flexible and less resilient to delays. Baggage fees are higher. Sometimes they work. Just look at the whole package.
Have a strict (but realistic) valuation for miles and use them opportunistically. Most importantly, avoid redeeming credit card points on their travel portals. That is just a fixed valuation per mile, and a cash-back card would've likely returned more money.

I rarely take taxis or rideshare - really! I take public transportation or rent a car.
For public transit from airports, there is sometimes two tiers of services: express and regular. The express service can be a bad value, multiple times more expensive and not much faster.
For rental cars, my target budget is $60/day USD
- Rental car companies charge in 24 hour blocks. Aim to maximize use of that time. Always reserve the car for the full 24 hour block. Don't return late or without fuel.
- Hotels especially in the US charge exorbitant parking fees. Look for street parking. Don't risk parking tickets.
- Midsize cars cost just few dollars more than economy cars but handle significantly better. Anything larger is less fuel economical and usually unnecessary.
- For extended rental periods (3+ days), renting off-airport can save money. Airport rentals can incur significant daily fees. For shorter rental periods, that's not worth the hassle.
- I always buy refundable rates. Plans can change, and rates can drop before pickup.
- Make sure you are adequately insured, both collision & liability.
- Manual transmission driving skills can save even more money. I'll invest in that someday.

I usually stop in a supermarket for some supplies on arrival.

Breakfast - For breakfast I eat a bread and maybe a piece of fruit. This is easy for me as I love bread and trying what the local bakery has on offer. $2-4 USD
Lunch - For lunch I often eat a sandwich, either purchased or prepared myself. This is also more time efficient on packed itineraries. $3-7 USD
Dinner - I'll look for a solid local option. $7-15USD
Dessert - I love dessert. I also rarely buy dessert at a restaurant - it's usually not good and poor value. I'll get a treat mid-afternoon and/or go somewhere after dinner

Bottled water is a huge money drain. I often buy 1-1.5L plastic water bottles on arrival and refill it. Tap water is safe in many countries - no shame in refilling water from the hotel bathroom sink if necessary. If a country does not have safe drinking water, the bottled water is very cheap anyway.
Daily coffee can add-up especially if you drink multiple cups like me. I try to maximize use of complimentary hotel coffee or brew my own with the hotel kettle. In either case I'll carry coffee for the rest of the day in a travel mug.

Get information from the local tourist office, especially outside of major cities. Look for the old-fashioned wall of printed material at airports, train stations, and hotels. The local offices often assemble fantastic information that is all free - printed maps to create walking tours, unique facts, what food is in season, local events/festivals, etc. They love their city and know it best. Having printed material so you can put your phone away is an added bonus!
I never buy packaged tours. Not only does that take the fun out of travel planning (that's why you're here!), I have particular taste and pace preferences, and DIY is also always cheaper
I rarely use guides. If needed I try to read up on things beforehand.