Welcome to the Sparks Team.
I really sympathize with your dilemma. I AM stuck in a wheelchair and was having trouble finding any way to keep my heart healthy, too. I didn't want to end up like my poor mother-in-law who just quit even trying to get any exercise when it became difficult. Now all of her systems are slowly shutting down because of her lack of exercise. No way was I going to have that happen to me.
My oldest son, who also has MS and musr use a wheelchair, is able to use a stationary bike, as well as some of the other equipment at the gym. He goes after the early businessmen have departed, so he can get a bit of extra help if needed.
On the other hand, I can't get to the gym and needed some activities I can do on my own. For a while, I used a set of bike pedals on the floor, until spasticity in my 'bad' leg became a problem. I moved that bike thing up on the table and cranked away at it for 10 - 15 minutes. It did get my heart pumping, but some days it made my shoulders hurt and, yep, it got boring.
Then I discovered that I could use some of the activities on the kids' Wii Fit game. Now, I can 'box' for 20 or 30 minutes, or 'jog'. I get my heart pumping, and it's a lot more fun. I can also 'bowl', play 'ping-pong', 'baseball', and other 'active' games, all from my wheelchair.
Laughter is a greast addition to exercise, and, for me, makes a 'solitary' activity enjoyable. And when the kids get home, we play more games.