You must have Grub to dual-boot Ubuntu, but there are some other options that may fit your needs.
Answering your question:
The time that the grub boot screen is displayed can be modified, so you
can set the time to zero. Then you would have to have Windows set as your default OS and use the information on this forum for when you wish to select your Ubuntu OS. The information shared there includes some hackery, so I won't go into detail describing it here.
There is another option that could work as well. You could install Windows and Linux on separate hard drives, and set BIOS/UEFI to default to the Windows drive. Whenever you want to boot into your Linux drive, reboot and go into your BIOS/UEFI settings and select the Linux drive for a one-time boot. The exact steps to do this is dependent on the manufacturer of your motherboard, so I'm leaving out specific details.
Option 2:
Using a bootable flash drive might meet your needs. This requires you to change the boot order in the BIOS settings, but it is quite straightforward and is perfect for random tinkering. There is also much less of a chance of overwriting partitions, so that's a huge plus. Flash drives are cheap, so for $15 you could try out a half dozen different distros.
Option 3:
Use virtual machines to run on your computer. This is free using Oracle VirtualBox.
Option 4: Use WSL, Windows Subsystem for Linux. This is a new project that allows a Windows user to experience most of the features of Linux directly on their Windows machine.
Option 5: Get a Raspberry Pi Zero and ssh into it via the serial port. This is awesome for a true Linux environment on the go.
Note: I personally use all of these methods. Each has its advantages and disadvantages.