Two options for the wall, but you would have to select which specific plants because we can only guess at your climate. You love colour and both of these can cater for that:
1. Plant soft hedging shrubs with variegated foliage immediately in front of the wall and have a flower bed immediately in front of that. N.B. trees will have too large gaps in between, although it is highly unlikely fruit trees would damage the footings (foundations) of the wall. You could use soft fruit shrubs or aromatic herbs as part of your hedge.
2. Construct a trellis in front of the wall, attached to it higher up. You have a choice of flowering climbers to choose from for most climates, you could even put in a mixture to extend the flowering season considerably.
Some of the ideas for the manhole covers are very good. The centrepiece does not have to be a bird bath or bird feeding table although I do like the idea. You have to suit your own taste when deciding what to put there. Instances may be a lightweight statue or figurine, a weathervane, even a water feature so long as it is moveable when needed.
One observation to add: that lawn looks very sickly. Is there any grass among the weed and moss? Do you know the reason? May I suggest early treatment, as leaving it in that state for too long will incur the expense of total replacement.