There are a lot of ways around this. First let me respond to your attempts.
A couple notes on your usage. The kanji 御飯 and 美味しい aren't that common in Japanese usage. ご飯 and おいしい respectively are more common. (Though it's not something people will shoot you down for, either).
Note your question asked "How was dinner?" so you need to respond using a past tense: おいしくなかった
Onto your samples:
This sounds awkward to a Japanese person. 少し isn't the word to use here; that has more to do with quantitative things (amount, time, etc.) Think if I said "This is a a pinch of horrible." It'll get the point through, but not the most elegant of answers. Here you would use あまり.
That will get you your "Kinda bad, but not awful" feeling. Maybe more like "It wasn't that good" feel.
This is awkward as well. Again, you can't really use 少し here, and おいしすぎない isn't really a phrase that is used. It's literally saying "This is a pinch of delicious, but not too delicious," which doesn't make much sense in English as well. Maybe something like:
おいしかったと言えばおいしかったけど、何かが足りなかったな。I guess you can say it was good, some I felt like something was missing.
Other ways of saying things are OK could be:
まあまあでしたよ。It was OK. (neither good nor bad)
まあまあよかったかな？ It's was well... good. (leaning slightly to good)
微妙だったな。That was meh. (a bit colloquial)
Or you could give it a rating:
70点くらいかな？ About 70 points?
及第点ってところ。 It got a passing grade (but just barely).
中の上だと思います。 Top of the middle class.