I've had very good luck with Arapahoe Rental (formerly Grand Rental Station in Loveland). They have everything from excavators to core drills to bounce houses for kids. The last time I returned a rental, they noticed that I had paid too much and refunded the difference, without me saying a thing. It's also nice that they offer hourly rentals (with a 4-hour minimum). This would have been a 5-star review if they had the exact core drill bit size I needed recently, which a competitor did have.