I've never had HAL do any "fun" things for me as a single cruiser, but I recommend them. You'll find more people in your age group on HAL, both single and married. If you sail during non-peak times (fewer kids!), HAL has some cruises with 140% single supplements. I've never felt out of place on HAL.
I sailed with HAL once - Westerdam Jan 05. They seem to have a reputation for having a mature following. Knowing that, I gave them a try. Although their single supplement seems the most reasonable of the lines, I will probably not sail with them again. At 56, I felt like a youngster. Most of the passengers were old, in age as well as spirit. I like to be around old folk who are lively and have a since of humor - not stuffy people. One positive thing about sailing solo on HAL is their single/solo cruiser luncheons, which seems more comfortable that the typical single gatherings, which I have never attended, but understand them to be like little cocktail socials. In May 05, I did a transatlantic sailing on RCCL's Jewell of the Seas. The passenger demographics were probably about the same, but the entertainment was much more upbeat than on the Westerdam. Also, the elderly passengers were fun and energetic - even those in wheelchairs. On the Jewell senior citizens were at the disco kicking it up to the sounds of the Village People. On the Westedam, the passengers would probably have been appalled by Village People music. And, contrary to a previous comment, I am not a novice cruiser. I have cruised about 15 times since 1998, usually during the Nov - Feb timeframe.