If you are big into Aikido.... similar arts would be Jiu-jitsu (O'Sensei originally studied this before developing Aikido). There are many ryu of Jiu-Jitsu (JJ).... The JJ I currently practice is Brazilian Jiu-jitsu (BJJ). BJJ instruction varies according to the instructor, but it has applications for sport, self-defense, fitness, gi, and no-gi. Some schools focus on certain aspects more than others.
One of the main 'proverbs' my instructor uses is "take what they give you." I think that goes well in-line with the idealogy of Aikido (correct me if I am wrong).
Judo (another descendant of Jiu-Jitsu) is also a good choice. It can be for self-defense, sport, and fitness as well. Different schools teach Judo differently. Some are highly competitive, others are more recreational. While BJJ focuses mostly on newaza (ground techniques) Judo focuses mainly on throws.
Judo/Jiu-Jitsu is good because it will develop a very good sense of balance which will also help your aikido technique. Because some of the more dangerous techniques are removed from both arts, full-contact sparring can be done regularly and safely, which really help you to apply your techniques and get a feel for a real fight/attack.
Make sure to check out the dojo first though. Every instructor is different. Some Judo/Jiu-Jitsu gyms/dojos are really hard-core and brutal. Others can be highly sociable. Make sure you do the one that is right for you, and make sure the instructor has your safety in mind. Bumps and bruises do happen, even the occasional over-extension of a joint, but there shouldn't be a frequent or high rate of injury. That is a bad sign.
If you are looking for spiritual enlightenment, do yoga or go to church.
If you just want to try a new art for the fun of martial arts and aren't concerned with combat effectiveness, by all means, just look around.
Tae Kwon Do and karate schools are everywhere. Their instruction varies, some can be awesome, others can be piss-poor. But I know that most people who practice those arts aquire tremendous flexibility. A lot of Tae Kwon Do black-belts come to my Jiu-Jitsu gym to learn grappling techniques.
I think understanding basic grappling is very beneficial, as most people will seek to grab you in an attack so that they can control. By learning grappling, even if you don't want to grapple in a real encounter, you should aquire the skills to stay upright and break free of grips.
If you are having trouble deciding, check out some books.
The Idiot's Guide to Martial Arts is a pretty broad overview of lots of different arts.
If you are interested in Brazilian Jiu-Jitsu, rent some of the early UFC's (especially 1-5), check out The Gracie Way by Kid Peligro, or Mastering JuJitsu by Renzo Grace and John Danaher.
If you are interested in Judo look up Jigoro Kano and Masahiko Kimura on the internet. Both are influential in the world of Judo and may help you can insight on the art. Kano was the founder of Judo. Kimura is undoubtedly one of the best if not THE best player of all time.
Hope this helps.
All in all, keep an open mind. I hope you have success on your journey.