Just simple press ups can provide a work out for your chest, as a lot of people give themselves a gentle warm up with these.

one I used to do ages ago was & sorry if this doesn't make any sense but I'll do my best

have a clock in front of you for this & do 10 - 15 press up what ever you feel comfortable with in one minute, this you'll probably do in 15 - 25 seconds say & leave the rest of the minute to rest

then repeat the above ie 10 - 15 press ups in next minute & again what ever time you have left over use this to rest

Do this for 10 minutes so all in you do 100 - 150 press ups, I found this a really good way of working my chest, although others found it really easy & didn't do anything for them

**** If you're back starts hurting stop though ***

With working the stomach there are plenty of other exercises which "shouldn't" put too much strain on you back eg lay flat on you back & lift you feet 2 inches off the floor and hold them there then lift to approx 6 inches & return to 2 inches & do this 10 - 15 times or again whatever feels comfortable

again if you're back starts hurting then stop.

HTH