Instead of continuing straight across Mercer Island from the east, you can loop around Mercer Island for an extra 9.0 miles. Cross the bridge to Mercer Island, turn left on East Mercer Way, becomes West Mercer Way. At end of island loop, turn left at SE 32nd Street, right at 60th Ave SE, then left to return to the I-90 Trail to Seattle.
(Views are better if you do the loop counter-clockwise.)
(Instead of taking the I-90 trail to close out the loop you can take N. Mercer Way and 24th to bypass the traffic and pedestrians at the park & ride and in the lid park.)
Bill Thorness has a detailed route description on page 92 of his new book Biking Puget Sound.