Setting project tempo

You could just enter it in the tempo dropdown in the view menu

FWIW, here's how i like to set tempo in a new project:
 
- Disable Stop at Project End in Track View Options if it isn't already so you can run the transport in an empty project.
- Start playback with no click sounding, count off a couple-few measures in your head, and stop the transport on the next downbeat.
- Shift+M to open Set Measure/Beat At Now, and enter that measure and beat.
- SONAR will set the tempo to make the specified beat fall on the absolute Now time where you stopped the transport.
 
This gives very accurate results in a new project with low latency where the transport start/stop is almost instantaneous. Note that SONAR will also insert a matching tempo at the beat you set. It won't matter unless you go changing tempos later, but you can delete it if necessary.
 
This works especially well when setting a tempo for someone else. You can just ask them to count or beat a tempo and start/stop the transport on their count.

There's also tap tempo, although that still isn't continuously variable.

chuckebaby
the whole right side of my Custom module is full of Tempo commands, which give me quick access to tap tempo, set tempo and meter changes <---- that last one I consider part of tempo options seeing how tempo and key signatures are the basis of any material you are laying out.

That's a great tip! I particularly like the quick access to tap tempo aspect.

cpkoch
Will the tempo setting approach set the tempo for all of the notes in a piece or just the note set for "now time"

Shift-M allows you to specify the timeline position for where the now time cursor is sitting, and this will only assign that one position. Depending on how much drift there is in a song, you can sometimes get away with only needing this every few measures or even more (for pieces with tight timing or recorded to a drum machine), but many pieces do not vary enough that you need to do more often than the down beat on each measure (but a few may require this).
 
That said, the tempo map is only important if you intend to add MIDI (soft synths) tracks, since they will run and are clocked to the project tempo. If working only with audio, the tempo map is not as critical, unless using plugins that are tempo-based (delays, etc.).
 
As you work with covers a lot, the easiest way to tempo map is to set the project tempo "close" and scoot the audio to the right so that the first downbeat lands on a measure marker. Shift-M that (to lock that measure to that downbeat), then start working down the timeline to assign downbeats that are drifting off the timeline.
 
As an aside, a nifty "utility" program that is very good with giving tempo information, key, etc. is a program called "RiffStation" which is more geared for guitarists, but has a lot of utilities anyone can use. That was made *free* earlier this year, and the web site for it still shows it as available. It is worth grabbing that and registering it, even if you may not use if right away. A some point it is rumored that it will disappear, but it habitually is very good with tempo detection. Edit: Not even sure if you have to register it... it seems to be "just free" after I clicked the download link.