Are there any easy spots to put in to do a quick afternoon paddle? I'd really like to see the Saint Elena Canyon, can you put in right next to the canyon and float through and back? I don't want to have to hire a shuttle.
Unless you want to piddle around up or downstream of the canyons (not too appealing for the effort involved vs. the return), any real trip is going to involve a shuttle, with one exception. That is to put in at the Santa Elena takeout and paddle upstream to Fern Canyon then float back to the takeout.
The full Santa Elena trip is not easy and may be quite difficult depending on water conditions. The Santa Elena put-in is outside the park.
Boquillas is a long trip down to the La Linda takeout and I really have no idea as to how feasible it would be to paddle into Boquillas and then turn around against the river. It would depend on water flow. Might be easy, might be impossible. The La Linda takeout is outside the park and doing that canyon would involve the greatest amount of driving and coordination to achieve.
Mariscal is a 10 mile float with almost no issues, but you do need two vehicles or a shuttle. If you park along the river in remote places like Talley or Solis, and are obviously gone from the area, be aware that vehicle break-ins do occur and are not necessarily an infrequent event.
Don't forget you need permits

and, depending on the current interpretations

of hysteria that is called 'border security'

may need your passport.